LOANED   TO 
UNIVERSITY   OF    CALIFORNIA 

DEPARTMENT    OF     MECHANICAL    AND     ELECTRICAL     ENGINEERING 
FROM     PRIVATE    LIBRARY    OF 

C.    L.    CORY 
1930 


ELECTRIC  MOTORS 


THEIR  ACTION,  CONTROL  AND 
APPLICATION 


BY 


FRANCIS  B.  CROCKER,  E.M.,  PH.D. 
I? 

PROFESSOR     OF     ELECTRICAL     ENGINEERING,     COLUMBIA     UNIVERSITY, 
PAST    PRES.    A.  I.  E.  E.,    MEM.    BRITISH    INST.    OF    E.    E. 


AND 


MORTON    ARENDT,  E.E. 

ASSISTANT    PROFESSOR   OF    ELECTRICAL   ENGINEERING,    COLUMBIA 
UNIVERSITY,    MEM.    A.  I.  E.  E. 


NEW    YORK 

D.  VAN    NOSTRAND    COMPANY 

23  MURRAY  AND  27  WARREN  STREETS 
I9IO 


II 


Engineering 
Library 


a    *      •    i  «  *      *«•>*•*•         *       • 
*    °  .f  '    *-•*•<."*         *    '*4 


Engineering 
Library 


COPYRIGHT,  1910,  BY 
D.  VAN  NOSTRAND   COMPANY 


Stanbopc  ipress 

F.   H.   GILSON     COMPANY 
BOSTON.     U.S. A 


PREFACE. 


THE  design  and  construction  of  electrical  apparatus  are  covered 
fairly  well  by  existing  literature,  the  books  on  such  subjects  being 
very  numerous,  and  many  of  them  are  comprehensive  and  authori- 
tative. On  the  other  hand,  the  operation  of  electrical  machinery 
Jias  received  comparatively  little  attention.  This  latter  fact  ap- 
pears to  be  anomalous  when  we  consider  that  there  are  undoubt- 
edly several  hundred  users  for  every  designer  or  constructor  of 
such  apparatus,  because  each  builder  supplies  a  large  number  of 
customers.  Hence  the  authors  have  endeavored  to  supply  infor- 
mation that  may  be  useful  to  those  who  operate  or  are  interested 
in  the  .operation  of  electric  motors.  Included  among  these  are 
electrical  engineers  who  install  or  run  electric  power  plants,  man- 
agers of  manufacturing  or  other  establishments  in  which  electric 
devices  are  employed,  as  well  as  students  and  others  who  desire  to 
acquaint  themselves  with  the  working  of  various  kinds  of  electric 
motors  and  their  application  to  useful  purposes. 

The  subject  is  necessarily  technical  because  it  involves  not  only 
the  mechanical  factors  speed  and  torque  but  also  the  electrical 
quantities  voltage,  current  and  flux.  Moreover,  any  or  all  of  these 
five  quantities  may,  in  fact  usually  do,  vary  and  are  affected  by 
other  quantities  or  conditions.  Hence  the  problems  must  be  ana- 
lyzed and  solved  with  thoroughness  to  obtain  results  of  real  value 
and  cannot  be  properly  treated  in  a  popular  manner.  Nevertheless 
care  has  been  taken  to  introduce  and  explain  each  step  or  result  as 
clearly  as  possible,  and  to  illustrate  each  case,  when  feasible,  by  a 
specific  numerical  'example  based  upon  standard  commercial  motors. 

The  general  method  herein  adopted  is  an  outgrowth  of  the 
course  of  lectures  on  electric  motors  and  their  applications  given  in 
Columbia  University.  It  is  based  upon  the  consideration  of 
counter  e.m.f.  and  its  relation  to  impressed  e.m.f.  as  the  important 
criterion  of  motor  action.  This  point  of  view  is,  of  course,  not 
original,  but  it  is  claimed  that  the  conception  is  more  explicitly  and 

iii 

789478 


iv  PREFACE. 

widely  applied  than  heretofore.  Furthermore,  this  idea  brings 
together  the  motor  and  generator  so  that  they  may  be  regarded  as 
identical  except  for  slight  differences  easily  seen,  and  our  knowledge 
concerning  one  is  applicable  to  the  other.  The  plan  of  treatment 
also  links  voltage  with  speed,  and  current  with  torque,  since  in 
general  they  are  respectively  proportional.  Thus  we  consider  one 
pair  of  quantities  at  a  time  instead  of  four.  The  synchronous  a.c. 
motor  differs  so  radically  from  the  d.c.  type  that  the  treatment 
must  be  modified,  but  even  in  this  case  a  similar  standpoint  is 
adopted  as  closely  as  possible. 

Throughout  the  book  references  are  given  to  United  States  and 
foreign  patents  as  well  as  articles  and  books  in  which  may  be 
found  further  descriptions  of  the  various  machines  and  methods 
considered.  Those  portions  of  the  A.  I.  E.  E.  Standardization 
Rules  relating  to  electric  motors  have  been  extracted  verbatim  and 
put  together  as  Appendix  A. 

The  authors  gratefully  acknowledge  their  indebtedness  to  Messrs. 
J.  H.  Morecroft,  A.  G.  Popcke,  L.  W.  Rosenthal,  A.  H.  Timmer- 
man,  E.  H.  Waring  and  G.  B.  Werner  for  valuable  information  and 
to  F.  L.  Mason  for  assistance  in  proof  reading.  They  also  take 
this  opportunity  to  thank  the  Crocker- Wheeler,  Electro-Dynamic, 
General  Electric,  Wagner  and  Westinghouse  Companies  for  illustra- 
tions and  data  of  apparatus  manufactured  by  them  and  discussed 
in  this  book. 

January  5,  1910. 


TABLE   OF   CONTENTS 


CHAPTER   I 

PAGE 

INTRODUCTION i 


CHAPTER  II 
TYPES  OF  MOTORS  AND  ADVANTAGES  OF  ELECTRIC  DRIVE. 


CHAPTER  III 

ACTION  OF  SHUNT  MOTORS 10 

CHAPTER  IV 
SHUNT-MOTOR  STARTING  BOXES 33 

CHAPTER  V 

SHUNT-MOTOR  SPEED  CONTROL  BY  VARIATION  OF  ARMATURE  VOLTAGE 38 

CHAPTER  VI 

SPEED  CONTROL  OF  SHUNT  MOTORS  BY  VARIATION  OF  FIELD  CURRENT 46 

CHAPTER  VII 

SPEED  CONTROL  OF  MOTORS  BY  VARIATION  OF  FIELD  RELUCTANCE 67 

CHAPTER  VIII 

MULTIPLE- VOLTAGE  SYSTEMS  OF  MOTOR  SPEED  CONTROL 74 

CHAPTER  IX 
DIRECT-CURRENT  SERIES  MOTORS 94 

CHAPTER  X 

CONTROL  OF  DIRECT- CURRENT  SERIES  MOTORS 109 

CHAPTER  XI 
COMPOUND- WOUND  MOTORS 118 

v 


yi  TABLE   OF   CONTENTS 

CHAPTER  XII 

PAGE 

ALTERNATING-CURRENT  MOTORS  —  INTRODUCTION 123 

CHAPTER  XIII 

SYNCHRONOUS  ALTERNATING-CURRENT  MOTORS 131 

CHAPTER  XIV 

POLYPHASE  INDUCTION  MOTORS 165 

CHAPTER  XV 

STARTING  OF  POLYPHASE  INDUCTION  MOTORS 194 

CHAPTER  XVI 

SPEED  CONTROL  OF  POLYPHASE  INDUCTION  MOTORS 204 

CHAPTER  XVII 

SINGLE-PHASE  INDUCTION  MOTORS 213 

CHAPTER  XVIII 

COMMUTATING   ALTERNATING-CURRENT   MOTORS 238 

CHAPTER  XIX 

SERVICE  CONDITIONS  AND  APPLICATIONS  OF  ELECTRIC  MOTORS 265 

APPENDIX  A 
STANDARDIZATION  RULES,  ELECTRIC  MOTORS 273 

INDEX 287 


Electric  Motors,  their  Action 
and  Control. 


CHAPTER   I. 

INTRODUCTION. 

An  electric  motor  is  a  machine  which  converts  electrical  power 
into  mechanical  power.  In  function,  therefore,  it  is  the  exact 
converse  of  the  dynamo-electric  generator.  On  the  other  hand, 
identically  the  same  machine  may  be  and  often  is  employed  to  per- 
form either  function,  which  fact  is  known  as  the  reversibility  of  the 
dynamo-electric  machine.  In  the  earlier  periods  of  their  develop- 
ment, however,  the  two  machines  were  usually  regarded  as  quite 
different  in  character  and  were  constructed  on  wholly  different 
lines. 

Strange  to  say,  the  motor  historically  precedes  either  the  magneto- 
or  dynamo -electric  generator.  Barlow's  wheel  of  1823,  the  first 
electric  motor,  was  similar  in  construction  to  Faraday's  disc  of  1831, 
which  was  the  original  magneto-electric  generator.  The  Jacobi 
electric  motor  of  1838  was  large  enough  to  propel  a  boat  carrying 
fourteen  passengers  at  three  miles  per  hour,  and  Page  in  1851 
constructed  a  car  driven  by  a  i6-horsepower  electric  motor  at 
nineteen  miles  per  hour.  These  as  well  as  other  electric  motors  of 
those  times  were  far  more  powerful  and  were  regarded  as  more 
practical  or  more  promising  than  the  contemporaneous  magneto - 
electric  generators.  The  Pacinotti  ring  of  1861,  the  prototype  of 
modern  armatures,  was  primarily  intended  to  be  used  in  a  motor, 
although  the  inventor  suggested  that  it  could  also  be  employed  to 
generate  electric  currents. 

All  of  these  early  electric  motors  depended  upon  primary  bat- 
teries for  their  supply  of  electrical  energy,  and  it  was  found  that  the 

1 


2  ELECTRIC   MOTORS,    THEIR   ACTION   AND   CONTROL. 

-  *xfi    »"*a  »**°*»  |        =    A."     "I  — 

cost  of  operation  was  excessive  for  any  considerable  power,  especially 
with  the  low-efficiency  motors  and  crude  forms  of  battery  then 
available.  The  result  was  that  the  motor  had  to  wait  while  the 
generator  was  being  developed  to  commercial  success,  which  cannot 
be  said  to  have  really  begun  before  1880.  Even  then  the  electrical 
energy  produced  was  used  entirely  for  arc  and  incandescent  light- 
ing. In  fact,  it  was  not  until  about  1887  that  central  stations  with 
their  systems  of  distribution  had  become  sufficiently  large  and  well 
regulated  so  that  the  use  of  electric  motors  was  encouraged  or  even 
permitted  except  in  a  few  isolated  cases. 

The  electric  light  having  been  practically  introduced  and  more 
or  less  generally  established,  inventors,  manufacturers,  also  those 
who  produced  electrical  energy,  turned  some  attention  to  electric 
power,  which,  from  about  1888,  has  been  a  prominent  part  of  electri- 
cal engineering,  including  railway  as  well  as  stationary  motors.  The 
former  type,  also  the  induction  and  synchronous  alternating-current 
motors,  began  to  be  commercially  introduced  about  that  time  or  soon 
after.  Since  this  comparatively  recent  epoch  the  progress  of  electric 
power  in  all  its  branches  has  been  at  an  extraordinarily  rapid  rate 
and  with  most  far-reaching  results,  unequaled  by  any  other  art  or 
industry  in  anything  like  the  same  period  of  time. 

Relation  between  Generator  and  Motor.  — Either  a  dynamo- 
electric  generator  or  a  motor  may  be  regarded  as  made  up  of  a  cer- 
tain number  of  centimeters  of  wire  located  in  a  magnetic  field  of 
given  density  in  lines  per  square  centimeter.  The  former  machine 
will  generate  e.m.f.  when  the  wire  moves;  the  latter  machine  exerts 
torque  if  current  flows  in  the  wire,  that  being  the  essential  distinction 
between  the  two.  We  may  have,  for  example,  a  generator  (sepa- 
rately excited)  producing  full  e.m.f.  with  no  current  in  its  armature 
and  we  may  have  a  motor  exerting  full  torque  with  its  armature 
prevented  from  turning,  but  we  cannot  have  a  generator  without 
motion  or  a  motor  without  armature  current.  In  practical  opera- 
tion, however,  the  generator  has  current  flowing  in  the  wire  so  that 
torque,  opposed  to  driving  force,  is  also  exerted;  and  in  the  motor 
e.m.f.,  counter  to  energizing  current,  is  set  up  by  the  motion  of  the 
wire.  Hence  either  machine  while  working  develops  both  e.m.f.  and 
torque,  the  only  difference  between  them  under  these  conditions  being 
the  fact  that  this  e.m.f.  is  positive  with  respect  to  current  in  the 
generator  and  negative  in  the  motor,  while  torque  is  negative  with 


INTRODUCTION.  3 

respect  to  motion  in  the  generator  and  positive  in  the  motor.  It 
follows  therefore  that  electrical  power  is  positive  in  the  generator  and 
mechanical  power  is  negative,  whereas  electrical  power  is  negative 
in  the  motor  and  mechanical  power  positive.  In  fact  the  exact  func- 
tion of  these  machines  is  expressed  in  the  above  statement,  which 
means  that  they  convert  mechanical  power  into  electrical  power  and 
vice  versa.  These  distinctions  in  function  or  action  do  not,  however, 
involve  any  necessary  difference  in  the  construction  of  generators  and 
motors.  As  already  stated,  identically  the  same  machine  is  equally 
operative  for  either  purpose  because  the  dynamo-electric  machine  is 
perfectly  reversible.  In  practice,  motors  and  generators  are  made 
somewhat  differently,  but  merely  with  respect  to  details  of  form  or 
connections,  so  that  they  will  be  more  convenient  for  the  special  uses 
to  which  they  are  applied.  As  a  matter  of  fact  motors  differ  among 
themselves,  railway  and  stationary  types,  for  example,  fully  as  much 
as  they  differ  from  generators. 

While  these  differences  in  construction  are  for  the  most  part  mere 
matters  of  adaptability,  the  operation  of  generators  is  radically 
unlike  the  operation  of  motors.  The  former  are  almost  universally 
driven  at  constant  speed  by  steam  engines,  gas  engines,  turbines  or 
other  sources  of  mechanical  power.  Of  course  in  practice  the  speed 
varies  somewhat,  but  this  is  very  undesirable  and  avoided  as  much 
as  possible  by  the  most  careful  design  as  well  as  adjustment  of 
governors.  The  few  cases  in  which  the  speed  variation  is  large, 
as,  for  example,  the  driving  of  a  generator  from  the  axle  of  a  railway 
car,  involve  serious  mechanical  as  well  as  electrical  difficulties,  spe- 
cial and  often  complicated  auxiliary  apparatus  being  employed. 

On  the  other  hand,  the  speed  of'electric  motors  is  very  commonly 
variable  or  adjustable,  the  range  in  many  cases  being  from  zero  to 
a  maximum  in  either  direction,  as  in  railway  or  elevator  service,  and 
ratios  of  three  or  four  to  one  or  higher  are  common  in  factories, 
machine  shops,  etc.  The  means  and  methods  used  to  accomplish 
such  speed  variation  constitute  an  important  branch  of  engineering, 
and  it  is  the  particular  purpose  of  this  book  to  discuss  this  subject 
of  motor  control.  In  those  applications  for  which  constant  speed 
is  desired,  the  motors  may  depart  somewhat  from  this  condition 
owing  to  their  own  action,  which  matter  will  also  be  given  special 
attention,  because  it  is  often  of  practical  importance  to  reduce  or 
allow  for  these  undesired  changes  of  speed. 


CHAPTER   II. 

TYPES  OF  DIRECT-CURRENT  MOTORS  AND  ADVANTAGES 
OF  ELECTRIC  DRIVE. 

MANY  kinds  of  electric  motors  are  in  use,  each  having  its  char- 
acteristics of  design  and  operation.  In  general  electric  motors  are 
divided  into  those  of  the  direct-current  and  alternating-current 
groups,  which  in  turn  may  be  subdivided  into  particular  types. 
Alternating-current  motors  will  be  discussed  later.  For  the  present 
attention  is  confined  to  direct-current  motors,  the  types  of  which  are 
as  follows: 

DIRECT-CURRENT  MOTORS. 

Type  Operative  Characteristics. 

Shunt-wound  motors Starting  torque  obtainable  in  actual  practice  is  50 

to  100  per  cent  greater  than  rated  running  torque, 
and  fairly  constant  speed  over  wide  load  ranges. 

Series-wound  motors Most  powerful  starting  torque  of  any  electric 

motor,  speed  varying  greatly  (inversely)  with 
load  changes. 

Compound-wound  motors .  . .  Compromise  between  shunt  and  series  types. 

Differentially  wound  motors .  Starting  torque  limited,  for  which  reason  these 

motors  are  rarely  if  ever  used  practically.  They 
are  nevertheless  interesting  scientifically  because 
their  speed  can  be  made  almost  absolutely  con- 
stant for  load  changes  within  rated  capacity. 

No  attempt  is  made  herein  to  describe  the  design  or  construction 
of  electric  motors  except  special  features  relating  to  speed  control. 
The  general  subject  of  motor  structure,  mechanical  and  electrical, 
is  treated  in  a  number  of  standard  works  as  listed  at  the  end  of  this 
chapter,  and  a  reasonable  knowledge  of  such  matters  is  assumed. 
For  example,  the  various  parts  of  motors,  their  names,  forms  and 
relation,  are  supposed  to  be  understood. 

The  sole  function  of  electric  motors  is  to  drive  some  other  machine 
or  device,  but,  as  the  number  of  such  applications  is  practically 
infinite,  their  field  of  utility  becomes  almost  universal.  Some  of 
the  prominent  uses  include  the  driving  of  cars,  pumps,  fans,  machine 

4 


TYPES   OF   DIRECT-CURRENT  MOTORS.  5 

tools,  looms,  printing  presses,  hoisting  apparatus,  grinding  and 
polishing  machines,  etc.,  etc.  Before  taking  up  the  discussion  of 
motor  action  and  control  it  will  be  well  to  consider  the  advantages 
thereby  secured. 

ADVANTAGES  OF  ELECTRIC  DRIVE. 

1.  Saving  in  Power.  — This  is  generally  the  first  point  to  be 
considered,  but  it  is  by  no  means  the  most  important,  as  the  cost  of 
power  in  manufacturing  is  rarely  more  than  i  to  3  per  cent  of  the 
cost  of  the  finished  product,  the  expenditure  for  labor  alone  being 
usually  many  times  greater.     It  is  a  fact,  however,  that,  due  to  the 
absence  of  belting  and  shafting  losses,  which  are  usually  40  to  60  per 
cent  of  the  total  power  required  to  drive  the  various  machines,  the 
saving  is  considerable.     Furthermore,  the  complete  cutting  off  of 
electric   current   whenever   an   individually   operated   machine   is 
stopped,  compared  to  the  large  practically  constant  loss  with  belt- 
ing and  shafting,  is  much  in  favor  of  the  former  method.     In  fac- 
tories and  similar  industrial  establishments  the  load  factor  or  average 
power  is  only  20  to  60  per  cent  of  the  maximum  or  total  amount 
required  when  all  machines  are  working  at  full  load.     The  losses 
with  electric  drive  correspond  to  the  usual  or  average  conditions, 
while  belting  and  shafting  losses  vary  somewhat  with  load,  but 
nearly  correspond  to  possible  or  total  capacity  of  the  plant.     This 
advantage  is  more  or  less  offset  by  the  losses  involved  in  the  double 
conversion  of  energy  from  mechanical  to  electrical  and  back  to 
mechanical  form,  but  in  most  cases  and  in  the  long  run  the  former 
method  does  effect  a  real  saving  in  power  consumption.     This  is 
practically  the  case  when  the  machines  to  be  driven  are  scattered; 
on  the  other  hand,  if  they  are  very  compactly  placed,  with  minimum 
distances  between  them,  the  saving  in  power  by  electric  drive  might 
be  little  or  nothing,  but  some  or  all  of  the  other  advantages  now  to 
be  stated  would  be  secured. 

2.  Larger  Prime  Movers.  —  There  is  a  practical  limit  to  the  power 
that  can  be  transmitted  by  belting  from  a  single  unit.     On  the  other 
hand  an  engine  or  turbine  may  be  directly  connected  to  an  electric 
generator  producing  10,000  horsepower  or  more,  the  whole  of  which 
can  be  readily  transmitted  and  subdivided,  also  combined  with  the 
output  of  other  generators.     Thus  the  power  of  the  individual 
prime  mover  can  be  increased  to  almost  any  extent  provided  it  is 


6  ELECTRIC   MOTORS,    THEIR   ACTION   AND    CONTROL. 

utilized  electrically.  Enormous  pumps  or  air  compressors  are 
almost  the  only  machines  except  electric  generators  that  are  adapted 
to  be  driven  by  engines  of  5,000  horsepower  or  more.  For  thou- 
sands of  other  power  applications,  the  size  of  prime  movers,  including 
steam  and  gas  engines  as  well  as  steam  and  hydraulic  turbines,  is 
practically  limited  to  units  of  moderate  size  if  dependent  upon 
mechanical  transmission  and  distribution  by  belting  or  gearing. 
Of  course  these  statements  do  not  apply  to  steam  vessels,  in  which 
many  thousands  horsepower  are  often  applied  to  a  single  propeller, 
this  being  a  case  of  direct  connection.  They  do  apply,  however,  to 
electric-railway  service,  for  which  one  very  large  engine  in  a  power 
house  may  replace  many  steam  locomotives.  The  advantages  of 
large  units  compared  with  small  ones  include  saving  in  first  cost, 
floor  space,  fuel  and  attendance. 

3.  Cost  of  Buildings.  — Heavy  overhead  shafting  is  not  required 
for  the  electric  drive,  hence  the  buildings  may  be  made  lighter  and 
cheaper  in  construction,  as  they  do  not  have  to  carry  the  large  extra 
weight.    Moreover  it  is  not  necessary,  as  with  long  lines  of  shafting, 
to  take  special  precautions  in  order  to  avoid  any  settling  which  would 
throw  them  out  of  alignment  and   cause   serious  friction   losses, 
vibration,  etc. 

4.  Cost  of  Equipment.  — The  relative  expense  of  equipping  a 
factory  with  electric  motors  or  with  belting  and  shafting  is  not 
usually  much  if  any  greater  for  the  former,  even  if  we  do  not  consider 
the  lower  cost  of  the  lighter  building  construction. 

5.  Arrangement   of  Machinery.  — The   use   of   electric   motors 
enables  the  various  machines  to  be  placed  in  almost  any  desired 
position.     It  is  not  necessary  that  they  should  be  parallel  or  placed 
in  rows,  or  at  any  particular  angle  or  distance  with  respect  to  each 
other;  whereas  for  belting  and  shafting  the  machinery  must  be 
arranged  in  a  very  particular  manner,  and  very  often  it  has  to  be 
located  where  the  light  is  poor,  or  accessibility  and  other  important 
features  must  be  sacrificed.     A  great  advantage  due  to  the  flexibility 
of  the  electrically  driven  machines  is  the  use  of  portable  equipments 
which  are  easily  made  up  and  operated,  so  that  the  tool  is  frequently 
brought  to  the  work,  as,  for  example,  when  a  portable  drill  is  brought 
to  a  heavy  casting  or  to  a  large  number  of  castings,  or  when  a  verti- 
cal slotter  is  applied  to  the  outside  of  a  large  casting  at  the  same 
time  that  the  interior  is  being  bored. 


TYPES  OF   DIRECT-CURRENT   MOTORS.  7 

6.  Clear  Head  Room.  — The   elimination  of  overhead   belting 
and  shafting  by  the  use  of  motors  gives  a  clear  head  room,  which 
enables  overhead  cranes  to  be  used  freely;  a  fact  which  results  in 
great  saving  of  time  and  labor  in  tb.e  bringing  of  the  work  to  the 
tools  or  removing  finished  pieces.     The  clear  head  room  also  gives 
better  illumination  and  ventilation.     In  fact  the  saving  in  cost  of 
proper    illumination   may   be    very   considerable   because   general 
instead  of  local  lighting  may  be  obtained,  whether  natural  or  arti- 
ficial.    Comparing   plate    A,   which   shows  the   appearance   of   a 
silk-weaving  shed  operated  by  belting  and  shafting,  and  plate  B, 
an  illustration  of  the  same  mill  in  which  the  looms  are  operated 
electrically,  the  great  advantages  regarding  head  room  and  illumi- 
nation are  apparent. 

7.  Cleanliness.  — The  dripping    of  oil  from  overhead  bearings 
and    shafting   is   a  constant    source  of  annoyance,   and   the  dirt 
thrown  out  from  belting  is  an  even  worse  enemy  to  cleanliness. 
The  agitation  of  dust  by  belting  and  shafting  keeps  it  in  con- 
stant circulation,  so  that  it  penetrates  everywhere  and  everything. 
This    is   an   especially   important   matter   in   printing  and  textile 
work. 

8.  Health  of  Employees.  —  On  account  of  the  better  ventilation 
and  illumination,  and  reduction  of  dust  and  dirt,  it  is  shown  by 
actual  experience  that  the  general  health  of  those  who  work  with 
electrically  driven   machinery   is  improved.     In  the   Government 
printing  office  at  Washington,  it  was  found  that  the  sick  list  was 
decreased   as  much  as  40  per  cent    after  the   electric  drive  was 
introduced. 

9.  Convenience  for  Detached  Buildings.  —  The  electrical  method 
enables  power  to  be  supplied  easily  and  economically  to  detached 
buildings  or  sections,  which  is  not  possible  with  belt  or  steam  trans- 
mission; therefore,  the  buildings,  like  the  machinery  within  them, 
can  be  located  for  general  convenience,  and  not  with  special  regard 
to  supplying  them  with  power.     This  subdivision  of  an  industrial 
establishment  into  a  series  of  detached  buildings  is  an  almost  abso- 
lute safeguard  against  total  destruction  by  fire;  and  is  thus  a  practi- 
cal guarantee  of  continuous  earning  capacity.     If  electric   power 
were  not  employed,  it  would  be  necessary  to   have  very  extended 
belting   and  shafting  connections,  involving'  great  losses  and  extra 
heavy  wall  construction,  or  a  number  of  small  power  plants  with 


8  ELECTRIC  MOTORS,    THEIR  ACTION   AND   CONTROL. 

a  larger  force  of  men  and  considerably  less  economical  in  oper- 
ation than  one  central  power  house. 

10.  Freedom  for  Growth.  — For   similar  reasons,   with   electric 
drive  it  is  a  simple  matter  to  extend  a  building,,  or  add.another  in  any 
direction,  whereas  shafting  must  be  installed  originally  large  enough 
to  allow  for  extension;  or  else  it  must  be  replaced  later;  in  which 
case  the  operation  of  the  existing  line  shafts  would  be  interfered 
with. 

11.  Reliability. — Shut-downs  or  delays   are  less  frequent   and 
less  serious,  because  an  accident  in  an  electrically  driven  plant 
usually  has  a  local  effect  only,  simply  interrupting  the  service  of 
one  or  a  few  machines,  while  with  belting  and  shafting  the  breaking 
or  slipping  off  of  a  belt,  or  the  failure  of  a  friction  clutch,  may  require 
the  shutting-down  of  a  whole  plant  or  a  large  section  thereof.     Fur- 
thermore the  time  required  for  repair  is  usually  less  with  electric 
power.     In  a  large  establishment  a  delay  of  even  a  few  minutes 
represents  a  considerable  item  in  wages,  and  in  addition  the  inter- 
ruption of  the  work  is  demoralizing.     It  might  be  argued  that  the 
central  power  plant  may  break  down;  this  is,  however,  just  as  likely 
to  happen  with  one  form  of  power  transmission  as  with  another.     In 
case  of  damage  by  fire  or  flood,  the  machinery  can  be  moved, 
rearranged,   reconnected  and   started  again  much  more  promptly 
with  electric  driving  than  with  belting  and  shafting. 

12.  Speed  Control.  — The  variation  of  speed  that  is  possible  with 
the  electric  drive,  and  the  convenience  as  well  as  the  wide  ranges  of 
control,  are  great  advantages  which  in  many  cases  are  sufficient  in 
themselves  to  dictate  the  adoption  of  electric  motors.     The  operator 
can  drive  the  machine  to  its  limit  of  capacity,  and  can,  on  the  other 
hand,  instantly  relieve  it  of  strain.     With  mechanical  drive  the 
methods  of  speed  control  are  more  limited  and  require  more  time 
to  operate  than  with  electric  motors.     The  shifting  of  the  belt  on  a 
cone  pulley,  or  the  throwing  in  and  out  of  different  sets  of  gears, 
takes  more  effort  than  the  simple  turning  of  a  controller  handle, 
which  can  be  placed  in  a  much  more  convenient  position  than  is 
usually  possible  with  the  mechanical  device.     The  result  is  that  the 
operator  makes  more  frequent  use  of  the  former  in  order  to  gain  even 
slightly  in  the  efficiency  or  rapidity  of  his  work.     For  example,  the 
cutting  speed  in  the  case  of  the  electric  drive  can  be  kept  absolutely 
constant  or  at  maximum  value,  whereas  mechanical  drive  cannot 


PLATE    A,  —  SILK-WEAVING    SHED    WITH    BELT    DRIVE. 


PLATE    B.  —  SILK-WEAVING    SHED    WITH    MOTOR    DRIVE. 


TYPES  OF   DIRECT-CURRENT   MOTORS.  9 

be  adjusted  as  quickly  or  as  closely,  the  steps  of  speed  variation 
being  much  greater.  The  saving  in  time  thus  obtained  is  consider- 
able and  correspondingly  reduces  the  shop  cost  of  the  article.  It  is 
the  particular  purpose  of  this  book  to  discuss  the  matter  of  electric- 
motor  speed  control. 

13.  Increased   Output. — Owing   to   its   many   advantages,   but 
especially  on  account  of  clear  head  room  for  crane  service  and  con- 
venient speed  control,  it  is  found  that  the  output  of  manufacturing 
establishments  is  in  most  cases  materially  increased  or  the  running 
expenses  decreased  by  the  introduction  of  electric  drive.     An  added 
output  of  20  or  30  per  cent  is  often  obtained  from  the  same  plant, 
which  in  itself  is  sufficient  to  make  the  difference  between  profit  and 
loss  in  carrying  on  a  manufacturing  business. 

14.  Overtime  work,  also  work  on  holidays  or  during  strikes,  may 
be  carried  on  conveniently  and  economically  with  a  portion  of  the 
machinery  or  even  with  a  single  tool,  because  a  small  engine  and 
generator  may  be  run  to  supply  the  electric  power.     On  the  other 
hand,  the  main  engine  and  the  whole  or  a  large  part  of  the  shafting 
and  belting  would  have  to  be  operated  in  order  to  supply  the  power 
by  the  ordinary  mechanical  transmission. 

15.  Noise.  — Rumbling  of  line  shafts  and  slapping  of  belts  are 
entirely  done  away  with  when  electric  drive  is  adopted. 


BIBLIOGRAPHY,   ELECTRIC   MACHINE   DESIGN. 

DIE  GLEICHSTROMMASCHINE,  Vols.  I.  and  II.     E.  Arnold.     1908. 

DIE  WECHSELSTROMTECHNIK,  Vols.  I-V.    E.  ARNOLD.    1904-1908. 

DYNAMO-ELECTRIC  MACHINERY,  Vols.  I  and  II.     S.  P.  Thompson.     1904. 

ELECTRIC  MACHINE  DESIGN.     Parshall  and  Hobart.    1906. 

ELECTRIC  MOTORS.    H.  M.  Hobart.    1904. 

ELECTRICAL  MACHINE  DESIGN,  Vols.  I  and  II.     J.  W.  Esterline.    1906. 

ELEKTRISCHE  GLEICHSTROMMASCHINEN.     J.  Fischer-Hinnen.    1904. 

THE  DYNAMO.    Hawkins  and  Wallis.    1903. 

THE  INDUCTION  MOTOR.    H.  B.  De  La  Tour.    1903. 

THE  INDUCTION  MOTOR.    B.  A.  Behrend.    1901. 


CHAPTER  III. 

ACTION  OF  DIRECT-CURRENT  SHUNT  MOTORS. 

LET  us  now  study  the  action  of  shunt-wound  motors  under  various 
conditions  of  load,  temperature,  speed,  etc.  It  is  well  to  consider 
first  what  occurs  due  to  the  conditions  existing  or  changing  within 
the  machine  itself,  by  its  own  action,  after  which  the  effect  of 
external  or  purposely  introduced  factors  will  be  explained.  To  make 
the  results  as  significant  as  possible,  standard  shunt-wound  machines 
have  been  selected  as  examples.  Three  typical  sizes  are  considered 
and  compared,  i.e.,  i,  10  and  no  horsepower.  The  exact  data 
concerning  these  machines  and  calculations  based  thereon  are  given 
in  Table  i  of  the  present  chapter.  It  is  to  be  remembered  that  the 
average  size  of  motors  is  less  than  that  of  generators,  several  of  the 
former  being  usually  fed  by  one  of  the  latter.  Hence  these  sizes 
represent  small,  medium  and  fairly  large  machines.  It  is  also  a  fact 
that  the  i  lo-horsepower  size  is  sufficiently  large,  so  that  still  larger 
motors  will  correspond  closely.  For  example,  the  efficiencies  of  the 
three  sizes  are  about  81,  86  and  93  per  cent,  respectively,  above 
which  last  figure  the  efficiency  would  increase  only  i  or  2  per  cent. 
Therefore  the  characteristic  differences  are  found  below  no  horse- 
power, and  these  machines  may  be  taken  to  represent  commercial 
practice  with  respect  to  shunt  motors. 

A  few  simple  tests  determine  the  fundamental  facts  from  which 
the  action  of  these  machines  under  almost  any  reasonable  conditions 
may  be  readily  calculated.  Most  of  the  tests  are  well  known,  but 
they  are  included  here  as  a  desirable  part  of  the  definition  of  these 
fundamental  quantities,  to  avoid  any  uncertainty  in  regard  to  them. 

It  is  assumed  that  the  construction  of  shunt  motors  is  already 
understood  by  the  reader,  the  present  book  being  confined  to  action 
and  control  of  this  and  other  types.  The  literature  of  dynamo-elec- 
tric generators  and  motors  is  extensive  in  regard  to  their  theory, 
design  and  construction,  there  being  many  works  in  which  these 
matters  are  fully  covered  but  their  operation  has  not  been  given 
the  attention  that  it  deserves. 

10 


ACTION   OF  DIRECT-CURRENT  SHUNT   MOTORS. 


11 


i.  The  Voltage  V,  for  which  the  motor  is  designed  and  at  which 
it  normally  operates,  is  assumed  to  remain  constant,  being  applied 
to  the  terminals  of  the  armature  and  field  circuits,  which  in  the  shunt 
type  are  in  parallel  (Fig.  i).  If  V  is  not  constant  it  should  be  main- 
tained so  (for  experimental  investigation)  by  inserting  a  rheostat 


Armature 
FIG.   I.  —  SHUNT-MOTOR    CONNECTIONS. 

which  can  be  adjusted  to  correct  any  variations.  This  voltage 
should  be  that  marked  on  the  manufacturer's  name  plate  and  is  gen- 
erally known  as  the  rated  voltage.  It  may  be  found  later  that  some 
other  voltage  is  preferable  in  order  to  obtain  a  different  speed  or 
other  resuU,  in  which  case  a  new  series  of  tests  should  be  made  at  the 
modified  voltage. 

2.  The  Total  Current  I  taken  by  the  motor  at  rated  load  is  also 
marked  on  the  name  plate.     This  may  be  found  later  to  differ  from 
the  current  at  which  the  rated  horsepower  is  developed,  or  it  may 
cause  heating  in  excess  of  the  limit  specified  in  (4).     In  either  case 
another  series  of  observations  should  be  taken  with  the  corrected 
current.     For  the  present,  however,  it  will  be  assumed  that  the 
rated  voltage  V  and  the  rated  current  /  are  both  correctly  given 
on  the  maker's  name  plate.     In  the  shunt-wound  motor  the  total 
current  /  is  the  sum  of  the  armature  current  Ia  and  shunt-field  cur- 
rent I8h. 

3.  The  Room  Temperature  t  is  taken  at  25  degrees  C.  in  standard- 
izing electrical  apparatus.*  If  it  differs  from  25  degrees  C.  allowance 
should  be  made. 

4.  The  Temperature  Rise  6°  permissible  in  the  armature  or  field 

*  Standardization  Rules,  Amer.  Inst.  Elec.  Eng.,  1907. 


12        ELECTRIC  MOTORS,    THEIR   ACTION   AND    CONTROL. 

is  50  degrees  C.  as  measured  by  increase  in  resistance  of  these  respec- 
tive windings.  This  gives  a  working  temperature  of  t  +  0  =75 
degrees  C.  =  J1,  at  which  the  machine  is  said  to  be  "hot"  in  contra- 
distinction to  "cold"  at  the  room  temperature  t.  To  determine 
whether  the  temperature  rise  6  is  within  the  limit  of  50  degrees  C., 
the  motor  is  supplied  with  the  rated  voltage  V  and  operated  with  suf- 
ficient load  to  draw  the  rated  armature  current  Ia  until  a  constant 
temperature  T  is  reached,  requiring  from  6  to  18  hours,  depending 
upon  the  size,  speed  and  ventilation  of  the  machine.*  The  resist- 
ance of  the  field  and  armature  circuits  is  measured  before,  during 
and  after  the  run,  as  explained  in  (5)  and  (6).  This  resistance  at 
25  degrees  C.  is  RQ  +  (.0042  R0  X  25)  =  i.io$R0,  and  at  75  degrees  C. 
it  is  R0  +  (.0042  R0  X  75)  =  1.315  RQ,  in  which  R0  is  the  value  at 
o  degree  C. 

Hence  the  resistance  at  working  temperature  or  "hot  resistance" 
is  as  1.315:  1.105  :  :  1-19  :  !>  °r  Z9  Per  cent  greater  than  the  "cold 
resistance."  If  the  increase  in  resistance  is  found  to  be  more 
than  19  per  cent,  the  standard  safety  limit  has  been  passed, 
but  if  found  to  be  less  than  19  per  cent,  so  much  the  better,  not 
only  for  safety,  but  also  for  constancy  of  speed,  as  shown  later  (see 
page  26). 

The  capability  of  a  motor  to  carry  current  and  develop  power 
depends  upon  temperature  of  the  air  and  other  variable  conditions. 
Hence  the  rating  of  motors  and  other  electrical  apparatus  is  some- 
what arbitrary,  but  it  is  based  upon  the  long  experience  and  consen- 
sus of  opinion  of  those  who  make  and  use  them.  The  A.  I.  E.  E. 
Standardization  Rules  are  followed  herein  and  are  given  so  far  as 
they  relate  to  motors  in  Appendix  A. 

5.  The  Field  Current  Ish  in  the  shunt  motor  is  determined  by  con- 
necting the  field  terminals  directly  to  the  supply  circuit,  the  voltage 
of  which  is  V.  This  should  be  measured  when  the  machine  is  "  hot," 
that  is,  after  the  run  specified  in  (4)  to  obtain  working  conditions. 
The  field  current  should  also  be  determined  with  the  machine 
"cold,"  before  the  run,  because  speed  variations  are  caused  by  the 
temperature  changes,  as  explained  later  (Chap.  Ill,  p.  26),  Further- 
more, with  both  values  known,  the  increase  in  resistance  and  the 
temperature  rise  may  be  easily  calculated.  The  shunt-field  resist- 
ance "hot"  Rsh  =  V  +  Ish  and  the  corresponding  values  cold  are 

*  Standardization  Rules,  Amer.  Inst.  Elec.  Eng.,  1907. 


ACTION   OF   DIRECT-CURRENT  SHUNT   MOTORS.  13 

R',h  -  F '+  I',h,  from  which  Rsh  -*•  £'8fc  =  7'^  -  7.fc.  With  tem- 
peratures of  75  degrees  and  25  degrees  C.,  respectively,  it  was  shown 
in  (4)  that  Rt+9  -j-  R't=  1.19,  hence  7'sA  =1.19  Ish.  In  any  case, 
however,  the  temperature  rise  in  degrees  C.  is: 

e  =  (238.1  +  <,)  (|i  -  i),  '     (i) 

in  which  /t  and  7^  are  the  initial  temperature  and  resistance,  while 
R2  is  the  final  resistance. 

6.  The  Armature  Resistance  Ra,  including  resistance  of  brushes 
and  brush  leads,  but  not  brush  contacts,  is  also  measured  "hot." 
Potential  difference  or  voltage  "drop"  due  to  the  brush  contacts, 
which  depends  upon  the  current  density,  should  be  measured  at  the 
rated  current  value  and  deducted  from  the  total  drop  in  the  armature 
circuit,  to  get  the  true  resistance  of  that  circuit,  or  that  quantity 
which,  multiplied  by  the  current,  gives  the  IR  drop.  The  nature  and 
value  of  drop  due  to  brush  contacts  is  discussed  later  in  the  present 
chapter.  The  armature,  before  it  has  time  to  cool  off  after  the  run 
specified  in  (4),  is  supplied  with  its  rated  current  7a,  but  is  not 
allowed  to  rotate,  under  which  condition  suitable  resistance  must  be 
inserted  in  series  to  compensate  for  the  absence  of  counter  e.m.f. 
The  total  drop  V  in  volts  across  the  armature  terminals  is  then 
measured,  also  the  drop  Db  due  to  the  brush  contacts,  and  we  have 
Ra  =  (V  —  Db)  -T-  7a.  The  armature  circuit  resistance  "cold,"  if 
entirely  of  copper,  is  then  R'a  =  Ra  -*-  1.19  =  84  Ra,  assuming 
"cold"  and  "hot"  temperatures  of  25  degrees  and  75  degrees  C., 
respectively.  As  a  rule,  however,  the  total  resistance  of  the  armature 
circuit  includes  that  of  the  carbon  brushes,  which  latter  has  a  nega- 
tive temperature  coefficient,  so  that  the  resultant  increase  between 
25  degrees  and  75  degrees  C.  is  about  15  per  cent,  or  R'a  =  Ra  + 
1.15  =  .87  Ra.  This  variation  in  armature  resistance  is  not  very 
important,  however,  as  it  will  be  shown  later  that  it  has  little  effect 
upon  the  efficiency,  regulation,  etc.,  of  the  machine.  Not  only  does 
this  statement  apply  to  shunt  motors,  but  it  is  true  generally  of  the 
various  types  of  electric  motors  and  generators,  as  affected  by  varia- 
tions in  armature  resistance  due  to  any  reasonable  temperature 
changes.  This  statement  should  be  understood  exactly  as  made  and 
should  not  be  taken  to  mean  that  armature  resistance  is  insignificant 
in  its  effect. 


14        ELECTRIC  MOTORS,    THEIR  ACTION   AND   CONTROL. 

COUNTER  E.M.F.   OF   SHUNT  AND   OTHER  MOTORS. 

Before  proceeding  with  the  various  problems  to  be  considered  in 
connection  with  electric  motors,  it  is  desirable  first  to  study  their 
counter  electromotive  force,  as  it  plays  an  exceedingly  important 
part  in  the  action  of  such  machines. 

The  counter  e.m.f.  of  a  motor  armature  is  the  e.m.f.  that  it  would 
develop  as  a  generator  when  operated  at  the  same  speed  with  the 
same  field  flux.  Hence  the  following  well-known  expression  for 
the  e.m.f.  of  a  d.  c.  generator  is  equally  applicable  to  both  cases. 

Let  $  =  flux  entering  or  leaving  the  armature  per  pole, 
n  =  total  number  of  inductors  on  the  armature, 
N  —  revolutions  per  minute, 
p  =  number  of  pairs  of  poles, 
b  =  number  of  circuits  in  parallel  in  the  armature  winding, 


p 

then  e  =  c.e.m.f.  of  motor  armature  =  -  (2) 

60  X  io8  X  b 

By  inspection  of  equation  (2)  it  can  readily  be  seen  that  with  $, 
n,  p  and  b  maintained  constant,  the  c.e.m.f.  varies  directly  with  N 
the  number  of  r.p.m.,  and  conversely  we  may  state  that  the  speed  of 
a  motor  varies  as  the  c.e.m.f.,  other  factors  being  constant.  This 
is  a  very  important  fact  in  studying  the  action  and  speed  control  of 
electric  motors,  especially  shunt  motors,  because  the  above-men- 
tioned quantities  remain  practically  constant  or  do  not  change 
greatly  in  this  type,  unless  purposely  varied.  In  series  or  compound- 
wound  motors  the  field  flux  usually  varies  considerably  with  the 
current  and  torque.  In  fact  in  a  lightly  loaded  series  motor  it 
increases  almost  directly  with  the  current.  Even  in  the  case  of  these 
machines  their  counter  e.m.f.  is  used  in  Chapter  XI  as  the  criterion 
in  determining  their  speed  variation  and  control.  This  funda- 
mental and  general  significance  of  the  counter  e.m.f.  of  electric 
motors  is  the  basis  of  the  method  of  treatment  set  forth  in  the 
present  book.  The  counter  e.m.f.  of  shunt  and  series  motors  or 
other  direct-current  types  can  be  determined  in  several  ways  which 
will  now  be  explained. 

i.  Experimental  Method  of  Determining  the  C.E.M.F.  —  The 
armature  shaft  may  be  fitted  with  a  heavy  flywheel,  so  that  the 
stored  energy  in  the  revolving  parts  is  great.  The  motor  is  then 


ACTION   OF  DIRECT-CURRENT  SHUNT  MOTORS.  15 

operated  without  load,  but  at  rated  speed  (i.e.,  that  corresponding 
to  rated  load)  by  introducing  resistance  in  its  armature  circuit  in 
order  to  reduce  slightly  the  voltage  applied  to  it,  while  the  field 
is  excited  with  the  proper  line  voltage  F.  When  the  rated  speed 
is  attained,  the  armature  circuit  is  suddenly  opened,  and  the  fly- 
wheel effect  will  cause  the  armature  to  maintain  almost  constant 
speed  for  a  short  time,  during  which  the  c.e.m.f.  can  be  measured 
by  a  voltmeter  connected  to  the  armature  terminals,  since  it  then 
becomes  the  e.m.f.  of  the  machine  acting  as  a  generator. 

2.  Determination  of  E.M.F.  from  Torque  of  a  Motor  or  Generator. 
—  The  shunt  field  circuit  is  connected  to  the  supply  conductors  to 
allow  rated  field  current  ISh  to  pass  through  it.  The  armature  is 
also  connected  to  the  supply,  sufficient  external  resistance  being 
inserted  so  that  only  the  rated  load  current  Ia  flows  through  its 
winding.  This  develops  a  torque,  but  the  armature  is  not  allowed 
to  rotate,  a  metallic  or  wooden  bar  being  clamped  to  the  pulley  or 
shaft  of  the  machine.  By  means  of  known  weights  or  a  spring 
balance  we  measure  in  pounds  the  pull  plus  the  friction  of  bearings 
and  brushes,  also  the  pull  minus  friction,  add  these  together  and 
divide  by  two.  The  result  multiplied  by  the  length  of  brake  arm  in 
feet  may  be  called  the  true  torque  (Tt)  because  it  is  the  full  amount 
developed  by  the  interaction  of  the  magnetic  field  and  armature 
current.  Of  course  the  weight  of  the  arm  or  lever  should  also  be 
eliminated.  The  pull  plus  friction  is  easily  found  by  forcing  the 
armature  to  turn  slightly  against  its  tendency  to  rotate,  the  pull 
minus  friction  being  measured  by  yielding  slightly  to  the  motor's 
torque. 

Then  at  any  speed  N  in  r.p.m.,  the  gross  power  developed  would 
necessarily  be  2  nTtN  foot-pounds  per  minute,  which  divided  by 
33,000  is  the  total  mechanical  horsepower  evolved  in  the  armature 
and  corresponds  to  the  indicated  horsepower  of  a  steam  engine. 
This  must  equal  the  electrical  horsepower  supplied  to  the  armature; 
hence 


EIa  2;r7W746         TtN 

=  —  ;  ,  or  E  =  -  —    =  -  -,  (3) 

33,000        746  33>°°0/a          7-034 

where  E  is  the  motor  c.e.m.f.  or  generated  voltage  at  any  speed  N. 

The  true  torque  or  turning  effort  of  a  motor  depends  upon  the 
armature  current,  the  number  of  armature  inductors  and  the  flux 


16         ELECTRIC  MOTORS,    THEIR   ACTION   AND   CONTROL. 

through  the  armature.  It  is  independent  of  the  speed,  being  equal 
to  Tt  =  KIa$>,  where  K  is  a  constant,  depending  upon  the  number 
of  poles,  effective  conductors,  etc.  This  gross  or  true  torque  includes 
not  only  the  effective  torque  developed  by  a  motor  at  its  pulley 
when  running,  but  also  the  torque  required  to  overcome  friction, 
windage  and  core  losses.  In  the  case  of  a  generator,  the  total  torque 
is  that  necessary  to  revolve  the  armature  and  overcome  friction,  etc. 
Hence  effective  motor  torque  +  (friction  +  windage  +  core  loss 
torque)  =  true  torque  =  generator  torque  —  (friction  +  windage 
+  core  loss  torque).  In  the  case  of  belt-driven  machinery,  the 
effective  torque  is  equal  to  the  difference  in  tension  on  the  two  sides 
of  the  belt  multiplied  by  the  radius  of  the  pulley  in  feet  plus  one- 
third  belt  thickness. 

It  might  be  thought  proper  to  multiply^  the  electrical  power  in 
equation  (3)  by  the  efficiency  of  the  motor  in  order  to  equate  it  with 
mechanical  power.  We  should  remember,  however,  that  field  cur- 
rent is  not  considered,  core  loss  and  windage  are  absent  when  the 
armature  does  not  rotate  and  friction  is  eliminated  by  the  described 
method  of  measuring  the  true  torque.  Hence  we  are  dealing  here 
with  ideal  conditions,  the  usual  practical  losses  being  eliminated. 

3.  Calculation  of  C.E.M.F.  —  The  use  of  Equation  (2)  to  cal- 
culate c.e.m.f.  has  already  been  explained.  The  quantities  involved 
in  that  equation  are  determined  by  the  designer  of  a  motor  or  genera- 
tor, but  all  of  them  are  not  usually  known  to,  or  readily  ascertainable 
by,  the  user  of  the  machine.  Hence  the  following  method  is  given 
because  it  employs  data  easily  obtained  by  the  simple  tests  already 
indicated  under  the  head  of  "  The  Armature  Resistance"  on  page  13. 
These  tests  can  readily  be  made  after  the  machine  is  in  practical 
service. 

In  the  armature  circuit  of  any  direct-current  motor  the  applied 
voltage,  F,  overcomes  three  factors,  namely,  resistance  drop,  brush- 
contact  drop  and  the  c.e.m.f.;  hence,  V  =  IaRa  +  Db  +  c.e.m.f.,  or, 
rearranging,  c.e.m.f.  =  V  -  (IaRa  +  Db).  (4) 

Brush-contact  drop  or  fall  of  potential  which  occurs  at  the  con- 
tacts of  brushes  and  commutator  being  small  has  often  been  wholly 
ignored  in  tests  and  calculations  concerning  dynamo-electric  ma- 
chinery. Nevertheless,  it  is  a  measurable  quantity  producing  an 
appreciable  effect  upon  the  speed  of  a  d.  c.  motor  as  well  as  upon 
the  external  voltage  of  a  d.  c.  generator.  As  its  value  does  not 


ACTION   OF   DIRECT-CURRENT   SHUNT   MOTORS.  17 

ordinarily  exceed  i  volt  for  each  contact,  it  is  common  practice  sim- 
ply to  assume  2  volts  for  both  brush  contacts  of  a  d.  c.  machine. 
This  assumption  is  open  to  two  criticisms :  first,  the  fact  that  even  the 
maximum  brush  drop  may  not  be  and  usually  is  not  quite  as  much 
as  2  volts,  and  second,  in  any  case  it  varies  somewhat  with  the  current, 
so  that  it  is  certainly  less  than  this  amount  at  light  loads. 

If  brush  drop  increased  directly  with  current  it  could  be  included 
in  the  armature  resistance,  giving  a  total  value  which  multiplied  by 
the  current  would  be  the  total  armature  drop.  This  would  be  most 
convenient  for  both  tests  and  calculations,  but  unfortunately  does 
not  accord  with  the  physical  facts.  Brush  drop  appears  to  be  the 
combination  of  a  fall  of  potential  which  is  fairly  constant  and  a  true 
resistance  drop  IR  directly  proportional  to  the  current.  Probably 
the  approximately  constant  fall  of  potential  is  in  the  nature  of  a 
c.e.m.f.,  the  phenomenon  being  analogous  to  that  of  the  arc.  In 
fact,  such  a  contact,  especially  in  the  case  of  a  carbon  brush,  may 
properly  be  regarded  as  an  incipient  arc.  This  is  true  even  under 
favorable  conditions,  and  with  a  poor  contact  due  to  dirt,  vibra- 
tion, roughness  of  commutator,  etc.,  the  arc  becomes  actual  and 
apparent. 

The  curve  in  Fig.  2  is  based  upon  the  results  of  actual  tests  made 
with  a  number  of  brushes  similar  to  those  employed  in  the  three 
sizes  of  motor  specified  above.  The  voltage  is  the  total  value,  includ- 
ing the  potential  difference  or  drop  at  both  positive  and  negative 
carbon  brushes.  It  increases  with  the  current  density,  being  prac- 
tically *a  rectilinear  function,  as  Fig.  2  shows,  but  is  not  directly  pro- 
portional thereto,  since  the  drop  is  i  volt  at  i  ampere  per  square 
centimeter  and  2  volts  at  6  amperes  per  square  centimeter,  these 
being  the  ordinary  limits  for  small  as  well  as  large  machines  at  rated 
load.  The  current  densities  for  larger  motors,  also  generators,  are 
usually  higher  than  for  small  machines,  but  are  not  adopted  as  desir- 
able, being  practically  necessary  because  in  large  machines  many 
amperes  must  be  carried,  with  a  reasonable  number  and  size  of 
brushes,  upon  a  commutator  of  moderate  dimensions.  For  ordi- 
nary calculations  not  requiring  very  accurate  results,  the  loss  of 
potential  at  brush  contacts  may  be  included  with  that  due  to  arma- 
ture resistance.  This  assumes  that  the  former  is  a  simple  resistance 
effect  like  the  latter,  which,  as  already  stated,  is  not  the  physical  fact, 
but  the  percentage  of  error  thus  involved  is  usually  small.  Accord- 


18        ELECTRIC  MOTORS,    THEIR  ACTION   AND    CONTROL. 

ing  to  this  assumption  the  total  lost  voltage  in  the  armature  circuit  is 
Ia  (Ra  +  Rc),  in  which  Ia  is  armature  current,  Ra  armature  resist- 
ance and  Rc  is  a  resistance  equivalent  to  brush  contacts.  Taking 
these  quantities  at  their  rated  values  for  the  typical  lo-horsepower 
shunt  motor  in  Table  I  on  page  20,  we  have  37  (.28  +  Rc)  = 
10.4  +  1.4,  from  which  Rc  =  1.4  -r-  37  =  .038  ohm.  If  now  this 
resistance  be  multiplied  by  the  current  in  the  armature  when  it  runs 


Ml 

'1.8 
1.6 
1.4 
1-2 
1.0 
.8 
.6 
A 
.2 

^^ 

X1 

•^ 

^x 

•S 

/* 

••* 

^x 

•s 

/ 

^* 

^X 

^ 

x" 

^ 

X 

•** 

X 

•^ 

/ 

/ 

/ 

/ 

123456 
Amperes  per  Sn-  Cm. 

FIG.  2. TOTAL   VOLTAGE   DROP   AT    BOTH  CONTACTS   FOR  CARBON  BRUSHES. 

free,  the  brush  drop  thus  calculated  is  only  2.3  X  .038  =  .09  volt. 
The  actual  brush  drop  found  by  test  is  .84  volt  when  the  armature 
runs  free,  hence  the  calculated  result  is  .84  —  .09  =  .75  volt  less 
than  the  experimental.  This  difference,  which  would  be  about  the 

same  for  other  normal  motors,  is  only  * — ,  or  J  of  i  per  cent  of  the  rated 

230 

terminal  voltage,  and  would  introduce  a  corresponding  error  in  cal- 
culations of  speed,  etc.  Ordinarily  this  error  would  be  insignificant, 
especially  as  it  disappears  as  rated  speed  is  approached.  On  the 
other  hand,  the  percentage  of  error  is  twice  as  great  for  ii5~volt 
motors,  and  in  the  case  of  speed  control  by  armature  rheostat  or  mul- 
tiple voltage  it  would  be  too  large  to  be  neglected  except  for  rough 
calculations.  For  example,  the  true  speed  of  a  motor  running  at 

one-eighth  of  rated  speed  would  be  about — ,  or  2.6  per  cent 

230 


ACTION   OF  DIRECT-CURRENT  SHUNT   MOTORS.  19 

less  than  that  calculated  by  using  the  assumed  brush-contact  resist- 
ance of  .038  ohm. 

This  error  can  be  almost  entirely  eliminated,  however,  by  assuming 
that  the  voltage  lost  at  brush  contacts  is  made  up  of  a  constant 
c.e.m.f.  independent  of  current,  combined  with  a  true  resistance 
drop;  that  is, 

Db  =  b  +  IaRb.  (5) 

Referring  to  Fig.  2,  it  is  seen  that  the  experimentally  determined 
drop  at  brush  contacts  is  represented  by  a  straight  line  for  all  current 
densities  between  i  and  6  amperes  per  square  centimeter.  Below 
i  ampere  per  square  centimeter  the  line  curves  downward,  but  if  the 
straight  position  were  prolonged  backward  it  would  intersect  the 
vertical  axis  at  .8  volt.  If  the  straight  line  thus  completed  be  adopted 
as  the  basis  of  calculations,  we  have  the  simple  numerical  relation- 
ship that  there  is  an  initial  loss  of  potential  due  to  brush  contacts  of 
.8  volt  at  zero  current  and  there  is  an  increase  of  .2  volt  for  each 
ampere  per  square  centimeter.  For  example,  the  drop  at  3  amperes 
per  square  centimeter  is  .8  +  (3  X  .2)  =  1.4  volts  and  so  on. 
This  assumption  introduces  no  error  whatever  for  currents  above 
i  ampere  per  square  centimeter,  and  below  that  value  the  error 
is  insignificant  because  the  current  never  falls  to  zero,  being  a  mini- 
mum of  .19  ampere  per  square  centimeter  at  no  load.  At  this 
limit  the  distance  between  straight  and  curved  lines  represents  only 
.1  volt,  which  would  be  negligible  in  all  practical  cases. 

Substituting  the  above  explained  values  in  (5)  we  have  for  the 
typical  lo-horsepower  motor, 

1.4  =  .8  +  37  Rb, 
from  which 

Rb  =  (1.4  -  .8)  -s-  37  =  .016. 

The  general  equation  of  the  motor  thus  becomes 

V  =  e  +  b  +  Ia  (Ra  +  Rb).  (6) 

This  form  expresses  the  same  facts  as  equation  (4)  but  has  the 
great  advantage  that  the  empirical  quantity  Db  is  eliminated,  a 
rational  quantity  IaRb  being  substituted.  The  value  of  Rb  like  that 
of  b  is  constant  for  a  given  motor  and  does  not  vary  much  for  motors 
of  normal  design,  hence  the  drop  in  volts  due  to  any  value  of  ar- 


20        ELECTRIC   MOTORS,    THEIR   ACTION   AND    CONTROL. 

mature  current  is  completely  represented  by  b  +  Ia  (Ra  +  Rb),  in 
which  7a  is  the  only  variable. 

Shunt-Motor  Problems.  — The  following  data  of  three  standard 
sizes  of  shunt  motors  are  given,  so  that  the  various  features  of  these 
machines  may  be  studied  and  the  efficiency,  effect  of  temperature, 
speed  regulation,  etc.,  may  be  calculated.  These  results  are  ob- 
tained by  actual  tests  of  the  completed  motors.  The  precise  signifi- 
cance and  in  most  cases  the  method  of  determining  these  data  are 
stated  in  the  preceding  chapter,  but  in  general  the  table  speaks  for 
itself  and  the  figures  given  are  found  by  simple  measurements  using 
voltmeter,  ammeter  and  speed  counter.  It  is  also  possible  to  assume 
or  predetermine  such  data  and  use  them  as  a  basis  for  calculations 
similar  to  those  which  follow. 


TABLE  I.  — TEST  DATA  OF  TYPICAL  SHUNT  MOTORS. 


l-H.P.  Machine. 

10-H.P.  Machine. 

1  10-H.P.  Machine. 

Rated  Voltage    V 

230  volts 

230  volts 

230  volts 

Rated  Current  / 

4  amps. 

38  amps. 

384  amps. 

Arm.    Current    at    Rated 
Load,  Ia  
Shunt  Field  Current,  Ish.  . 
Arm.  Resist  "  Hot,"  Ra  .  .  . 
Arm.  Resist.  "Cold."#'a.  . 
Field    Resistance      Hot,' 

Rsh                               •     • 

3.85  amps. 
.15  amp. 
3.1  ohms. 
2  ohms 

1506  ohms. 

37  amps. 
1  amp. 
.28  ohm. 
.  244  ohm 

230  ohms. 

380.  7  amps. 
3.3  amps. 
.0104  ohm. 
.009  ohm. 

70  2  ohms. 

No-load     armature      cur- 
rent, 1'a  
Speed  at  rated  load  
Speed  at  no  load  
Brush  Contact  Area,  Ab  .  • 
Current  Density  of  Brushes 
at  rated  load,  Sb  
Current  Density  of  Brushes 
at  no  load,  Sb  

.4  amp. 
1250  r.p.m. 
1310  r.p.m. 
3  sq.  cm. 

1.28 
.126 

2.3  amps. 
825  r.p.m. 
865  r.p.m. 
12  sq.  cm. 

3.08 
.19 

14.2  amps. 
585  r.p.m. 
595  r.p.m. 
72  sq.  cm. 

5.3 
.20 

Drop   due    to   brush   con- 
tacts at  rated  load,  Db  . 
Drop   due    to  brush   con- 
tacts at  no  load,  D'b 

1.05  volts 
.83  volt 

1  .  4  volts 
.  84  volt 

1.86  volts 
.84  volt 

In  the  above  table  the  potential  difference  or  voltage  drop  due  to 
the  brush  contacts  has  been  taken  from  the  curve  in  Fig.  2. 

Calculation  of  Speed  of  Shunt  Motor  Running  Free  (i -Horsepower 
Machine).  —  It  was  shown  that  the  speed  of  a  motor  is  directly  pro- 
portional to  the  c.e.m.f.  at  any  instant,  other  quantities  such  as  field 


ACTION   OF   DIRECT-CURRENT  SHUNT   MOTORS.  21 

current  and  flux  being   constant,  hence  the   ratio  between   rated 
load  speed  and  no-load  speed  (i.e.,  free)  is 

r.p.m.:  r.p.m.    :  :  c.e.m.f. :  c.e.m.f./.  (7) 

From  equation  (4)  c.e.m.f.  =  V  -  (IaRa  +  Db),  in  which  we 
substitute  the  values  of  V,  IaRa  and  Db  as  given  in  the  data  sheet 
and  obtain  c.e.m.f.  =  230  —  (3.85  X  3.08  +  1.05)  =  217.1  volts 
at  rated  load,  and  c.e.m.f.  =  V  -  (I'aR'a  +  D'b)  =  230  -  [(.4  X 
2.7)  +  .83]  =  228.10  volts  at  no  load.  Substituting  these  values 
of  c.e.m.f.  and  the  rated  motor  speed  in  equation  (7),  we  have  1250  : 
r.p.m  :  :  217.1  :  228.1.  Therefore  r.p.m./.  =  1314  r.p.m.,  which 
is  within  .3  per  cent  of  the  test  value  (Table  I)  given  as  1310  r.p.m. 
for  the  speed  at  no  load. 

Speed  Running  Free  (lo-Horsepower  Motor). — Rated  load  c.e.m.f. 
=  V  -  (IaRa  +  A>)  =  230  -  (37  X  .28  +  1.4)  -  218.2  volts. 
No-load  c.e.m.f.  =  V  -  (I'R'a  +  D'b)  =  230  -  (2.3  X  .244  +  .84) 
=  228.6  volts.  The  rated  speed  is  825  r.p.m.,  and  by  substituting  in 
equation  (7)  we  have  825  :  r.p.m./.  :  :  218.2  :  228.6;  whence  r.p.m./. 
=  863,  which  is  within  .3  per  cent  of  the  test  value  of  865  r.p.m., 
for  the  no-load  speed. 

Speed  Running  Free  (i  ro-Horsepower  Motor) . —  Rated  load  c.e.m.f. 
==  V  -  (IaRa  +  Db)  =  230-  (380.7  X  .0103  4-  1.86)  =  224.2  volts. 
No-load  c.e.m.f.  =  V  -  (I'aR'a  +  D'b)  =  230-  (14.20  X  .09  +  84) 
=  229  volts,  and  from  equation  (7),  585  :  r.p.m./-  :  :  224.2  :  229  or 
r.p.m.y  =  597  r.p.m.,  being  within  .3  per  cent  of  the  test  value  of 
the  no-load  speed  given  in  Table  I  as  595  r.p.m. 

These  calculations  assume  armature  flux  to  be  same  at  no  load  as 
at  rated  load,  which  is  not  usually  the  case  because  of  armature 
reaction,  as  explained  later  in  the  present  chapter.  The  close  agree- 
ment between  calculated  and  experimentally  determined  speeds  is 
therefore  partly  accidental.  The  effect  of  armature  reaction  in 
reducing  flux  and  increasing  speed  depends  upon  the  position  of  the 
brushes  but  would  usually  be  appreciable  and  in  some  cases  actually 
causes  the  no-load  speed  to  be  less  than  that  at  rated  load.  Never- 
theless a  rise  in  speed  tends  to  occur,  due  to  diminished  drop  in 
armature  resistance  and  brush  contacts  when  the  load  torque  and 
armature  current  are  reduced.  This  tendency  is  correctly  repre- 
sented by  the  values  calculated  in  the  three  examples  above.  In 
fact  the  speed  will  actually  vary  in  accordance  with  the  numerical 


22        ELECTRIC  MOTORS,   THEIR  ACTION  AND   CONTROL. 

results  obtained  unless  some  other  condition,  for  example  that  of 
armature  reaction  or  of  temperature,  is  also  changed.  It  is  better, 
however,  to  study  and  determine  each  influence  separately  and  then 
combine  them  to  ascertain  their  resultant  effect.  Such  is  the  method 
of  treatment  adopted  herein. 

It  is  obvious  that  any  change  whatever  of  armature  current, 
whether  from  rated  load  to  no  load  or  otherwise,  has  a  tendency  to 
cause  speed  variation,  the  amount  of  which  may  be  calculated  by  a 
similar  use  of  equations  (4)  and  (7),  substituting  the  proper  values 
for  speed,  current,  etc. 

The  above  calculations  of  speed  running  free  assume  that  arma- 
ture is  "cold"  (25  degrees  C.).  If  the  load  were  suddenly  thrown 
off  a  motor  which  had  been  operating  with  full  rated  armature 
current  for  several  hours,  the  armature  would  not  cool  immediately 
and  its  resistance  would  remain  at  practically  rated  value.  In  the 
case  of  the  lo-h.p.  machine,  for  example,  the  c.e.m.f.  would  then  be 
230  —  (2.3  X  .28  -f  .84)  =  228.5  instead  of  228.6  volts,  but  the  cor- 
responding diminution  of  speed  would  be  less  than  2V  of  i  per  cent, 
which  is  inappreciable.  If,  on  the  other  hand,  rated  load  be  suddenly 
applied  to  a  "cold"  motor,  the  speed  will  not  diminish  as  much  as 
if  the  armature  were  "  hot"  (75  degrees  C.).  In  this  case  the  c.e.m.f. 
will  be  230  —  (37  X  .244  +  1.4)  =  219.6  instead  of  218.2  volts. 
Hence  the  speed  will  be  .6  per  cent  higher,  or  830  instead  of  825 
r.p.m.,  but  this  difference  is  practically  insignificant.  The  effect  of 
temperature  upon  speed  is  discussed  further  on  p.  26. 

EFFICIENCY  OF  ELECTRIC  MOTORS. 

Determination  of  Efficiency  of  Motor  at  Rated  Speed  and  Load.  — 

The  Standardization  Rules  of  the  American  Institute  of  Electrical 
Engineers  (paragraph  313)  state:  "All  electrical  apparatus  should 
be  provided  with  a  name-plate  giving  the  manufacturer's  name,  the 
voltage  and  the  current  in  amperes  for  which  it  is  designed.  Where 
practicable,  the  kw-capacity,  character  of  current,  speed,  frequency, 
type  designation,  and  serial  number  should  also  be  stated."  From 
the  data  thus  given  the  approximate  or  "name-plate  efficiency"  of 
a  motor  can  be  determined  as  follows: 
From  Name-plate  of  i -Horsepower  Motor. 

Input  at  rated  load  =  230  X  4  =  920  watts.  Output  at  rated 
load  =  i  h.p.  =  746  watts. 


ACTION  OF  DIRECT-CURRENT  SHUNT  MOTORS.  23 

Hence  efficiency  being  the  ratio  between  input  and  output,  the 
name-plate  efficiency  of  the  i-h.p.  motor  =  746  -r-  920  =  81  per  cent. 

Calculation  of  Efficiency,  Using  Test  Values  (Table  I).  —In  de- 
termining the  efficiency  of  a  motor  we  take  the  motor  input  at  rated 
load  and  then  calculate  the  stray-power  and  other  losses,  using  the 
values  found  by  actual  test  and  given  in  Table  I.  The  difference 
between  input  and  the  total  losses  gives  the  output,  hence  the  ratio 
of  input  minus  losses  to  input  gives  the  motor  efficiency. 

The  stray-power  losses  of  the  i-h.p.  motor  running  free  (i.e.,  at 
1310  r.p.m.)  are  equal  to  the  armature  input  at  no  load  minus 
the   armature  no-load  copper  and  brush   losses;   that  is,  the  no- 
load  stray  power  losses  =  VI' a  -  (I'*R'a  +  I'aD'b)  =  230  X  .4  - 
(.4  X  2.68  +  .4  X  .83)  =  91.2  watts. 

The  remaining  losses  at  rated  load  are: 

Loss  in  Field  Copper  IshV  =  .15  X  230  =34.5  Watts 
Loss  in  Armature  Copper  Ia2Ra  =  3.85*  X  3.08  =  45.64  Watts 
Loss  in  Brush  Contacts  /0A>  =  3-^5  X  1.05  =  4.04  Watts 


84.18  Watts. 

If  to  this  we  add  the  no-load  stray  power  of  91.2  watts,  the  total 
loss  is  84.18  +  91.2  or  175.4  watts.  The  motor  output  is  equal  to 
the  input  minus  losses.  The  input  by  test  (Table  I)  is  230  volts 
and  4  amperes,  that  is,  920  watts;  hence  the  output  is  920  —  175.4  = 

744.6  watts,  and  the  efficiency  by  definition  equals  — -   -  or  81.0  per 

920 

cent;  so  that  in  this  case  the  efficiency  by  calculation  is  exactly  equal 
to  that  by  name-plate  determination.  Ordinarily  (as  shown  by  the 
10  and  no  h.p.  examples  which  follow)  there  is  a  slight  differ- 
ence because  the  former  is  based  upon  purely  electrical  data  while 
the  latter  depends  upon  a  brake  or  other  test  of  actual  mechanical 
power  developed. 

The  assumption  that  the  stray  power  at  no  load  is  the  same  as  at 
rated  load  is  not  absolutely  correct,  since  it  will  be  lower  at  rated 
speed,  which  is  from  2  to  5  per  cent  less  than  with  the  motor  running 
free,  but  the  error  introduced  by  this  assumption  is  practically 
negligible,  as  will  be  proven  in  the  following  case  of  the  lo-h.p. 
motor. 


24        ELECTRIC  MOTORS,    THEIR   ACTION   AND    CONTROL. 

/ 

Determination  of  Efficiency  of  lo-Horsepower  Motor. 

Output  from  name-plate       10  X  746 

Name-plate  efficiency  = = — 

Input  from  name-plate        230  X  38 

D  =  85-3  Per  cent- 
8740 

The  calculation  of  efficiency  of  the  lo-h.p.  motor  is  similar  to  the 
foregoing  example  of  the  i-horsepower  machine,  but  the  stray-power 
losses  will  be  corrected  for  speed. 

The  stray-power  losses  of  lo-h.p.  motor  running  free  (i.e.,  at 
865  r.p.m.)  equal  the  no-load  armature  input  (230  volts  and  2.3 
amperes)  minus  the  no-load  armature  copper  and  brush  losses; 
that  is: 

Stray  power  at  no  load  =  230  X  2.3  —  (2.32  X  .244  +  2.3  X  .84) 
=  526  watts.  At  rated  load  the  motor  is  running  at  a  slightly  lower 
speed  of  825  r.p.m.,  hence  the  stray  power  will  be  less  because  the 
eddy  current  constituent  varies  as  the  square  of  the  speed,  and  the 
several  losses  due  to  hysteresis,  windage  and  friction  may  be  assumed 
to  vary  directly  as  the  speed.  In  ordinary  machines  of  this  size  the 
stray-power  losses  are  usually  divided  as  follows :  50  per  cent  due  to 
windage  and  friction,  25  per  cent  due  to  hysteresis  and  25  per  cent 
due  to  eddy  currents.  Hence,  75  per  cent  of  the  stray-power  losses 
vary  as  the  speed,  and  25  per  cent  vary  as  the  square  of  the  speed. 
The  stray  power  corrected  for  change  in  speed  from  865  to  825  r.p.m. 
or  4i  per  cent  will  be  (.955  X  .75  X  526)  -f  (.955*  X  .25  X  526)  = 
495  watts.  If  the  stray  power  had  been  assumed  to  have  the  same 
value  at  no-load  speed  as  at  rated-load  speed,  the  error  introduced 
would  therefore  be  526  —  495  =31  watts,  or  about  .4  per  cent  of 
8740  watts,  the  rated  input.  This  difference  is  so  small  that  it  may 
generally  be  neglected  in  practical  problems.  Furthermore  there 
is  in  most  cases  a  rise  in  stray-power  losses  as  the  load  increases. 
These  are  called  "load  losses,"  being  partly  due  to  larger  mechani- 
cal forces  and  therefore  friction,  also  to  augmented  hysteresis  and 
eddy  currents  because  of  altered  distribution  of  flux  which  is  crowded 
into  certain  portions  of  the  armature.  The  assumption  of  the 
higher  figure  for  stray  power  would  tend  to  cover  load  losses  which 
are  difficult  to  determine. 


ACTION  OF  DIRECT-CURRENT  SHUNT  MOTORS.  25 

The  losses  at  rated  load  in  addition  to  stray  power  are  as  follows: 

Loss  in  Field  Copper  IshV  =    i    X  230  =  230.    Watts 

Loss  in  Armature  Copper  PaRa=  $f  X   .28  =  383.3  Watts 
Loss  in  Brush  Contacts      IJ^b^  37    X    1.4    =    51.8  Watts 

665.1  Watts 

This  amount,  added  to  the  corrected  stray-power  value  gives  a 
total  loss  of  495  +  665  =  1160  watts.  Hence  the  output  is  equal 
to  the  input  (230  X  38  =  8740  watts)  minus  this  loss,  that  is, 
8740  —  1160  =  7580  watts. 

The  efficiency  is,  therefore,  7580  -*•  8740  =  86.8  per  cent.  A 
comparison  of  calculated  output  (7580  watts)  and  rated  output 
(10  X  746  =  7460)  shows  that  the  former  is  136  watts  greater;  so 
that  the  manufacturer  is  on  the  safe  side  when  the  motor  is  rated  to 
give  10  horsepower,  and  this  is  as  it  should  be,  overrating  of  ma- 
chinery being  bad  practice.  In  other  words  10.18  h.p.  are  actually 
developed  at  rated  input. 

Effect  of  Armature  Resistance  upon  Speed  of  Shunt  Motors.  — 
The  principal  and  instantaneous  cause  of  shunt  motor  speed  varia- 
tion with  changing  loads  is  the  varying  armature  current  and  conse- 
quent varying  armature  drop  ( =  IaRa) ;  hence  the  reason  for  making 
the  resistance  of  the  armature  (Ra)  as  low  as  possible.  This  cause 
of  speed  change  is  shown  by  a  consideration  of  the  typical  zo-h.p. 
motor.  Assume  its  armature  to  be  "hot"  (75  degrees  C.)  and 
let  us  determine  the  speed  change  due  to  variations  of  armature 
current  alone.  From  Table  I  we  have  the  following  test  values: 
Ra  =  .28  ohm,  brush  drop  at  no-load  .84  volts,  at  rated  load  1.4 
volts,  and  speed  at  rated  load  825  r.p.m.,  with  armature  current  Ia 
of  37  amperes  and  terminal  voltage  V  of  230. 

According  to  equation  (4)  the  c.e.m.f.  at  rated  load  with  arma- 
ture "hot  "  but  running  free  is  230  —  (.28  X  2.3  +  .84)  =  228.5 
volts.  Hence  from  equation  (7),  taking  the  c.e.m.f.  at  rated  load 

from  page  21,  we  have  r.p.m.  (free)  = X  825  =  864;  that  is, 

218.2 

the  speed  changes  from  825  to  864,  amounting  to  39  r.p.m.,  or  4$  per 
cent.  Thus  there  is  a  speed  rise  of  4$  per  cent  solely  on  account  of 
diminished  armature  drop  (including  brush  contacts)  when  the 
rated  load  is  removed  from  this  lo-h.p.  motor.  The  effect  is  the 
same  as  if  the  voltage  supplied  to  the  armature  were  raised 


26        ELECTRIC  MOTORS,   THEIR  ACTION  AND   CONTROL. 

about  4i  per  cent.  In  fact  the  available  voltage  is  actually  increased 
to  that  extent.  This  calculation  does  not  take  into  account  the  effect 
of  armature  reaction  which  tends  to  counteract  more  or  less  the 
speed  variation  determined  above,  as  shown  a  little  later. 

Effect  of  Temperature  Changes  upon  Speed  of  Shunt  Motors.  — 
Heating  of  armature  affects  the  speed  only  to  a  slight  extent,  and 
may  be  practically  neglected,  as  already  shown  on  page  22.  For 
example,  in  the  case  of  the  lo-h.p.  motor,  the  "cold"  armature 
resistance  is  .244  ohm;  the  "hot"  armature  resistance  with  a 
temperature  change  from  25  degrees  to  75  degrees  C.  (i.e.,  50  de- 
grees C.  rise  being  permissible)  is  15  per  cent  greater  (see  p.  13)  or 
.28  ohm.  The  speed  alteration  at  rated  load  due  to  this  heating  is 
determined  as  follows: 

C.e.m.f.  with  armature  "cold"  at  rated  load  =  230  —  (37  X 
.244  4-  1.4)  =  219.6  volts,  the  c.e.m.f.  with  armature  "hot"  and  at 
rated  load  being  218.2  volts.  Hence  the  speed  at  rated  load  and 
with  armature  "cold"  is  219.6  -f-  218.2  X  825  =  830  r.p.m.  instead 
of  825  r.p.m.  when  the  armature  is  "hot,"  an  increase  of  .6  per 
cent,  which  is  not  material  in  most  practical  cases,  as  the  change  due 
to  varying  load  may  be  4  or  5  per  cent  as  shown  above. 

As  already  noted  on  page  22,  the  load  may  be  and  often  is  sud- 
denly thrown  off  a  motor  when  its  armature  is  "hot,"  and  conversely 
rated  load  is  often  applied  to  a  "cold"  armature.  In  each  case  it  is 
merely  a  question  of  fact  how  warm  the  armature  may  be.  But  the 
variation  in  resistance  is  only  15  per  cent  and  produces  little  prac- 
tical effect  upon  speed,  as  shown  above. 

Change  of  Speed  due  to  Heating  of  Field  Circuit.  —  The  allowable 
temperature  rise  in  the  field  winding  is  50  degrees  C.,  causing  a 
19  per  cent  increase  in  the  resistance,  as  shown  on  page  12.  Since 
the  rated  resistance  is  the  working  or  "  hot  "  value,  being  230  ohms 
for  the  shunt  field  of  the  typical  lo-h.p.  machine,  it  follows  that  this 
resistance  at  ordinary  temperature  (assumed  to  be  25  degrees  C.), 
herein  called  "cold"  resistance,  is 


*h 

—  *-  =  -^-  =  193.3  ohms- 
1.19      1.19 

Current  in  field  (hot)          I8h  =  —  =  —  =  i  ampere. 

V        230 

Current  in  field  (cold)  I'8h  -  =  —  -  —  =1.19  amperes. 

R'sh      193-3 


ACTION   OF  DIRECT-CURRENT  SHUNT  MOTORS.  27 

Hence  the  current  in  the  coils  cold  is  19  per  cent  greater  than  when 
the  latter  are  hot;  and  from  magnetization  curves  of  standard  types 
of  shunt  motors  a  rise  of  19  per  cent  in  field  m.m.f.  causes  an  increase 
of  about  4  or  5  per  cent  in  the  flux,  or  the  field  is  this  amount  stronger 
"  cold  "  than  "  hot."  With  the  other  conditions  (e,  b,  n  and  p)  constant, 
the  speed  (N)  will  vary  inversely  with  the  flux  (<£) ,  as  shown  in  the 
following  transposed  form  of  equation  (2) : 

3>nN  2p  e  X  io8  X  60  X  b 

e  = —  or  N  = • 

io8  X  60  X  b  <&n  2p 

With  the  flux  4  to  5  per  cent  stronger  when  the  field  winding  is 
cold  than  with  it  hot,  the  speed  is  4  to  5  per  cent  lower.  This 
variation  of  speed  with  heating  of  the  field  winding  is  an  objection- 
able characteristic  of  the  ordinary  shunt  motor  for  work  requiring 
almost  perfectly  constant  speed,  such  as  weaving.  It  can  be  overcome 
by  employing  a  field  so  highly  saturated  that  a  moderate  change  in 
field  current  produces  only  slight  flux  variation;  or  a  field  winding 
composed  of  wire  having  a  zero  temperature  coefficient  would  secure 
a  like  result.  Both  methods  are  costly,  especially  the  latter,  because 
the  only  available  materials  are  alloys  whose  resistivity  would  be 
much  higher  than  that  of  copper,  demanding  correspondingly  greater 
cross  section  of  wire.  On  the  other  hand,  it  may  happen  that  the 
former  plan  employing  a  field  approaching  saturation  is  desirable  for 
other  reasons,  such  as  improved  commutation,  so  that  the  total 
advantage  warrants  the  expenditure  for  additional  ampere-turns  in 
the  field  coils. 

It  was  explained  under  the  preceding  heading  relating  to  heating 
of  armature  that  rated  load  may  be  put  upon  a  "cold"  or  a  "hot" 
motor  or  may  be  thrown  off  either.  In  the  field  winding  the  full 
current  flows  whether  the  machine  is  loaded  or  not,  so  that  the  tem- 
perature of  the  former  simply  increases  with  the  time  of  operation 
until  maximum  is  reached. 

Hence  the  heating  of  shunt-field  coils  and  the  percentage  of  speed 
rise  occasioned  by  it  are  practically  the  same  whether  the  motor  is 
running  free  or  loaded.  The  same  is  approximately  true  of  ar- 
mature heating  by  eddy  currents  and  hysteresis  in  its  core.  On  the 
other  hand,  heating  due  to  armature  resistance  increases  as  the 
square  of  the  current  (PaR^  and  is  therefore  very  small  at  light 
loads.  At  rated  load  it  is  about  equal  to  the  core  heating  in  ordinary 


28        ELECTRIC   MOTORS,    THEIR   ACTION   AND   CONTROL. 

machines,  in  which  case  the  temperature  rise  in  the  armature 
would  be  about  one-half  as  great  running  free  as  at  rated  torque. 
It  has  already  been  shown  (page  26)  that  speed  change  due  to 
armature  heating  is  small,  the  r.p.m.  being  .6  per  cent  higher 
when  the  armature  is  "cold"  (25  degrees  C.)  than  when  "hot" 
(75  degrees  C.).  Hence  the  speed  would  be  .3  per  cent  greater 
when  the  armature  begins  to  run  free  (i.e.,  cold)  than  when  it  has 
been  running  unloaded  for  several  hours. 

Effect  of  Voltage  Variation  upon  Speed  of  Shunt  Motors.  —  The 
speed  changes  of  shunt  motors  due  to  their  own  action  have  been 
discussed  in  all  cases  on  the  assumption  that  the  voltage  sup- 
plied to  them  was  constant.  Such  constancy  is  the  desirable  con- 
dition to  be  maintained  or  approximated  as  closely  as  practicable. 
Nevertheless,  appreciable  variations  of  voltage  do  occur  even  on  the 
best  regulated  circuits,  and  may  often  become  very  considerable,  that 
is,  5  per  cent  or  more,  whether  from  central  station  or  isolated  plant. 
Fortunately  the  shunt  motor  is  not  very  sensitive  to  these  variations, 
the  percentage  of  speed  change  being  usually  less  than  that  of  the 
voltage  change.  In  actual  practice  the  former  is  usually  from  .6 
to  .8  of  the  latter;  that  is,  a  5  per  cent  rise  or  fall  in  voltage  will  cause 
the  speed  to  rise  or  fall  3  to  4  per  cent.  In  this  respect  the  shunt 
motor  is  far  less  susceptible  than  the  incandescent  lamp,  the  ordinary 
carbon  filament  type  changing  its  candle-power  about  6  per  cent 
when  the  voltage  is  altered  only  i  per  cent. 

A  shunt  motor  in  which  the  magnetic  circuit  is  considerably  below 
saturation  runs  at  nearly  constant  speed  even  if  the  voltage  varies 
widely.  This  is  because  the  flux  3>  varies  directly  with  the  voltage 
F,  which  in  turn  is  very  closely  proportional  to  e,  the  c.e.m.f.  in  the 

e  X  60  X  io8  X  b 

expression  r.p.m.  = derived  from  equation  (2), 

3>n  2  p 

so  that  any  change  in  the  latter  is  cancelled  by  a  corresponding 
change  in  the  former,  r.p.m.  remaining  constant.  On  the  other 
hand,  with  a  magnetic  circuit  completely  saturated  and  therefore 
constant,  the  speed  would  vary  directly  with  e  which  is  nearly  pro- 
portional to  the  supply  voltage  V  in  the  normal  shunt  motor  with  an 
armature  circuit  of  very  low  resistance. 

In  almost  all  practical  cases  the  magnetic  circuit  is  partially  sat- 
urated, and  in  order  to  determine  the  percentage  of  speed  changes 
that  will  be  produced  by  a  certain  percentage  of  voltage  change  F, 


ACTION  OF  DIRECT-CURRENT  SHUNT   MOTORS.  29 

it  is  necessary  either  to  arrive  at  the  result  empirically  by  actual  test 
or  to  calculate  it  from  the  magnetization  curve  of  the  machine,  which 
is  more  or  less  individual  for  each  design. 

It  is  convenient  for  this  calculation  to  employ  what  is  known 
as  the  percentage  of  saturation.  This  quantity  according  to  the 
A.  I.  E.  E.  Standardization  Rules  (F.  IV,  par.  58)  "may  be  deter- 
mined from  the  saturation  curve  of  generated  voltage  as  ordinates, 
against  excitation  as  abscissas,  by  drawing  a  tangent  to  the  curve  at 
the  ordinate  corresponding  to  the  assigned  excitation,  and  extending 
the  tangent  to  intercept  the  axis  of  ordinates  drawn  through  the 
origin.  The  ratio  of  the  intercept  on  this  axis  to  the  ordinate  at  the 
assigned  excitation,  when  expressed  in  percentage,  is  the  percentage 

of  saturation."     It  may  also  be  found  from  the  relation  p  =  i  —-•> 

in  which  p  is  the  percentage  of  saturation  and  /  is  the  saturation 
factor,  which  is  denned  by  the  same  Rules  (par.  57)  as  "the  ratio  of 
a  small  percentage  of  increase  in  field  excitation  to  the  correspond- 
ing percentage  increase  in  voltage  thereby  produced."  It  is  not 
necessary,  therefore,  to  determine  the  complete  saturation  curve  of 
the  machine.  It  is  sufficient  to  ascertain  the  percentage  rise  or  fall 
in  the  voltage  developed  by  the  machine  running  as  a  generator  on 
open  circuit  at  any  constant  speed  when  the  shunt  field  is  excited 
first  by  normal  voltage  V  and  then  by  the  voltage  V  ±  v,  in  which  v 
is  the  percentage  of  variation  of  V  in  any  particular  case.  For 
example,  if  the  saturation  curve  or  the  test  just  mentioned  shows 
that  the  voltage  generated  by  a  machine  rises  2  per  cent  when  the 
voltage  V  exciting  the  shunt  field  is  increased  5  per  cent,  then  its 
saturation  factor  7=5-7-2  =2. 5  and  its  percentage  of  saturation 

p  =  i  -    -  =  i  -      -  =  60  per  cent.     Ordinarily  these  quantities 
/  2.5 

are  referred  to  the  rated  or  normal  value  of  V,  but  may  be  based  upon 
any  other  selected  value.  This  percentage  of  saturation  represents 
the  extent  to  which  the  magnetization  approaches  saturation.  If 
the  armature  core  were  wholly  saturated  this  percentage  would  be 
100,  while  it  is  practically  zero  for  moderate  flux  densities.  This 
same  percentage  of  saturation  represents  the  ratio  between  speed  varia- 
tion of  a  shunt  motor  and  change  of  voltage  V  supplied  to  its  terminals. 
In  the  above  example,  therefore,  the  speed  would  rise  .60  X  5  = 
3  per  cent  when  the  voltage  increased  5  per  cent,  the  percentage  of 


30         ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

saturation  being  60.  At  100  per  cent  or  complete  saturation  the 
speed  rises  or  falls  exactly  the  same  percentage  as  the  voltage.  On 
the  other  hand,  at  zero  saturation  or  with  low  flux  densities  practi- 
cally proportional  to  the  excitation,  the  speed  would  be  constant. 

In  this  discussion  it  is  assumed  that  the  resistance  of  the  shunt- 
field  circuit  is  constant,  in  which  case  the  field  current  varies  directly 
with  the  voltage  V.  This  would  be  approximately  true  for  a  change 
of  a  few  per  cent  in  the  value  of  V,  which  is  all  that  usually  occurs  in 
practice.  Of  course  any  increase  in  V  does  tend  to  raise  the  tem- 
perature and  therefore  the  resistance  of  the  field  winding,  so  that  the 
shunt-field  current  would  not  increase  or  decrease  quite  as  rapidly 
as  the  voltage  V.  This  fact  makes  the  percentage  of  speed  varia- 
tion slightly  greater  than  that  stated  above.  This  effect  is  similar 
to  that  due  to  the  gradual  heating  of  the  field  which  occurs  even 
when  V  is  constant,  as  already  explained  on  page  26. 

The  definitions  of  percentage  of  saturation  and  saturation  factor 
quoted  above  from  the  A.  I.  E.  E.  Standardization  Rules  refer 
either  to  one  point  on  the  saturation  curve  at  which  a  tangent  is 
drawn  or  to  "a  small  percentage  of  increase  in  field  excitation." 
This  limitation  is  imposed  because  a  point  of  tangency  or  a  very 
short  distance  on  a  curve  may  be  regarded  as  a  straight  line.  The 
voltage  variations  occurring  in  practice  may  be  so  small  that  this 
assumption  is  correct,  but  often  they  amount  to  5  or  10  per  cent  or 
even  more,  and  cases  might  arise  in  which  the  voltage  may  be  acci- 
dentally or  purposely  varied  50  or  75  per  cent.  For  the  purposes 
of  our  problem,  which  relates  to  the  effect  of  such  variations  upon  the 
speed  of  shunt  motors,  it  is  sufficient  to  consider  only  the  range  of 
variation,  the  form  of  the  curve  between  the  limiting  points  being  of 
no  consequence.  Hence  the  machine  is  driven  as  a  generator  at 
any  constant  speed,  and  d  the  difference  in  voltage  developed  is 
measured  with  field  excited  by  voltages  V  and  V  ±v  respectively. 
It  is  not  necessary  in  this  case  to  limit  v  to  the  "small  percentage" 
stated  in  the  definition.  If  d  and  v  are  expressed  as  percentages  of 
the  initial  values,  then  v  +  d  =  f,  the  saturation  factor;  which  signi- 
fies simply  the  fact  that  in  order  to  raise  the  voltage  generated  by  a 
certain  percentage  it  is  necessary  to  augment  the  magnetizing  current 
a  greater  percentage.  For  example,  a  saturation  factor  of  3,  which 
is  an  ordinary  value,  means  that  a  4  per  cent  rise  in  generated  voltage 
demands  a  3  X  4  =  12  per  cent  increase  in  magnetizing  current  or 


ACTION  OF  DIRECT-CURRENT  SHUNT    MOTORS. 


31 


exciting  voltage  which  are  assumed  to  be  proportional  to  each  other 
in  a  shunt  machine,  because  temperature  and  resulting  resistance 
changes  are  gradual  even  when  they  occur.  In  making  the  test  to 
determine  d  for  a  given  value  of  v,  only  a  voltmeter  is  connected 
to  the  armature;  hence  the  driving  power  required  is  small,  and  the 
speed  may  have  any  reasonable  value,  provided  it  is  constant. 

The  above  method  of  calculating  the  speed  of  a  shunt  motor  at 
various  values  of  terminal  voltage  V  was  applied  in  the  case  of  a 
lo-horsepower  ii5-volt  General  Electric  shunt  motor.  The  results 
thus  obtained  are  given  in  Table  II  *and  are  compared  in  Table  III 
with  the  measured  speeds  noted  during  a  speed-load  test,  at  the  same 
values  of  V  as  employed  in  the  calculations.  A  portion  of  the  mag- 
netization curve  of  the  motor  was  carefully  determined,  and  from 
this  the  following  relations  required  for  the  calculations  were  deter- 
mined. V  represents  the  voltage  applied  to  the  motor  field  winding; 
Ish  the  shunt-field  current  =  V  -+  R8h;  e.m.f.  represents  the  open- 
circuit  voltage  obtained  when  the  motor  was  operated  as  a  genera- 
tor at  the  specified  speed,  and  is  naturally  proportional  to  the  field 
flux  <£  developed  by  the  corresponding  value  of  Ish.  The  per 
cent  saturation  at  rated  excitation  with  115  volts  was  determined 
directly  from  the  magnetization  curve. 


TABLE  II.— TESTS  TO  DETERMINE  PERCENTAGE  OF  SATURATION 
OF  A  10-H.P.  SHUNT  MOTOR. 


Terminal 

Shunt 

E.m.f.  at 

A, 

B, 

1-f, 

Voltage, 
V. 

Field 
Current, 
/.». 

970  r.p.m. 
00$. 

Change  in 
I*. 

Change    in 
e.m.f.  or  *. 

A 
per  cent 
Saturation. 

105  Volts. 

1  .5  Amps. 

109  Volts. 

-8.5% 

-  3.6% 

57.5 

110 

1.57 

111.2 

-4.3 

-  1.7 

60.0 

115* 

1.64 

113.1 

0 

0 

63.0 

120 

1.71 

114.8 

+  4.3 

+  1.5 

65.0 

125 

1.78 

116.4 

+  8.5 

+  2.9 

66.0 

*  Rated  voltage  and  field  current. 

The  formula  employed  to  calculate  the  various  speeds  of  the  motor 
at  different  values  of  V  is  in  its  simplest  form  as  follows: 

r.p.m.  atV=  r.p.m.  at  rated  V  {i  +  (A  -  B)},  (8) 

wherein  A  and  B  must  be  given  their  proper  signs. 


32        ELECTRIC  MOTORS,    THEIR  ACTION   AND   CONTROL. 


TABLE  III.  — MEASURED  AND  CALCULATED  SPEEDS  OF  A  10-H.P. 
SHUNT  MOTOR  WITH  LINE  VOLTAGE  (F)  VARIED. 


V. 

105  Volts. 

110  Volts. 

115  V. 

120  Volts. 

125  Volts. 

/« 

1.5  Amps. 

1.57  Amps. 

1.64 
Amps. 

1.71  Amps. 

1.77  Amps. 

/a 

R.p.m. 

II.  p.m. 

R.p.m. 

R.p.m. 

R.p.m. 

Amps. 

Test. 

Calc. 

Test. 

Calc. 

Test.* 

Test. 

Calc. 

Test. 

Calc. 

10 

906 

920 

938 

946 

970 

992 

996 

1010 

1024 

20 

894 

910 

926 

934 

958 

976 

984 

996 

1012 

40 

880 

892 

912 

916 

940 

956 

966 

986 

992 

60 

866 

878 

900 

902 

926 

938 

952 

976 

978 

75 

852 

864 

886 

888 

912 

926 

937 

962 

964 

*  Rated  voltage  and  field  current. 

The  agreement  between  measured  and  calculated  speeds  is 
remarkably  close;  such  differences  as  do  exist  are  so  small  that 
they  come  within  ordinary  errors  of  observation. 


CHAPTER    IV. 

SHUNT-MOTOR    STARTING   BOXES. 

IN  starting  shunt  and  compound-wound  motors,  no  trouble  is 
likely  to  occur  in  connecting  the  shunt-field  coils  to  the  circuit 
because  their  resistance  is  high.  The  difficulty  is  with  the  armature, 
its  resistance  being  very  low  in  order  to  obtain  high  efficiency  and 
good  speed  regulation,  as  already  shown.  If  a  low-resistance  winding 


2345 

Starting  Box  Contact  Points 

FIG.    3. — STARTING    SPEED    AND    CURRENT    CURVES. 


be  directly  connected  across  the  line  terminals,  the  current  would  be 
so  excessive  that  it  would  tend  to  injure  or  destroy  it.  When  stand- 
ing still  an  armature  generates  no  c.e.m.f.,  so  that  the  entire  voltage 
of  the  supply  circuit  would  have  to  be  consumed  by  the  fall  of 
potential  in  the  armature  resistance  and  brush  contacts  (IaRa  +  Db). 

33 


34        ELECTRIC   MOTORS,    THEIR  ACTION   AND   CONTROL. 

Theoretically  the  armature  current  would  rise  in  the  typical  10- 
horsepower  motor  to  a  value  Ca  =  (V  —  Db)  -f-  Ra  =  (230  —  1.4) 
-r-  .28  =  816  amperes  compared  with  rated  current  of  only  37 
amperes.  This  equation  is  obtained  from  equation  (4)  by  making 
c.e.m.f.  equal  to  zero.  Practically  the  current  would  not  reach  such 
an  extreme  value,  because  the  fuses  or  circuit-breaker  would  act  to 
prevent  it;  but  a  very  excessive  armature  current  would  flow  at  least 
momentarily  with  injurious  electrical  as  well  as  mechanical  effects. 

To  prevent  injury  and  at  the  same  time  to  obtain  gradual  accelera- 
tion, an  adjustable  rheostat,  commonly  called  a  "starting  box,"  is 
inserted  in  series  with  the  armature,  the  resistance  of  which  is  gradu- 
ally reduced  as  the  speed  increases.  As  a  rule,  starting  boxes,  unless 
otherwise  specified,  are  designed  to  allow  the  motor  to  draw  an  initial 
current  about  50  per  cent  greater  than  the  normal  value,  so  that  the 
machine  may  develop  ample  torque  to  start  under  load.  The  fol- 
lowing example  indicates  the  method  of  determining  the  various 
values  of  the  resistance  required  in  the  starting  box.  Assume  that 
the  lo-horsepower  motor,  the  data  of  which  have  been  given,  is  to  be 
started  from  rest  under  load,  that  the  current  and  "speeding  up" 
curves  desired  are  those  shown  in  Fig.  3.  The  current  curve  gives 
the  current'  values  desired  at  the  various  positions  of  the  contact 
arm  of  the  starting  box,  seven  in  all,  decreasing  gradually  from  50  per 
cent  above  normal  at  start  to  rated  current  at  rated  speed.  The 
speeds  at  the  corresponding  points  are  obtained  from  the  speed  curve 
in  Fig.  3,  and  are  tabulated  as  follows: 

SPEEDS  IN  STARTING  10-H.P.  MOTOR  UNDER  LOAD. 


Position  of  Contact  Arm. 

Armature  Current  Per  Cent 
of  Rated  Value. 

Armature 
Amperes. 

Speed  in 
R.p.m. 

1     All  resistance  in   ... 

150 

55  5 

o 

2                         

133 

49.3 

450 

3       

120 

44.5 

640 

4  

112 

41.7 

750 

5  

106 

39.5 

800 

6 

102 

37  75 

820 

7    All  resistance  out  

100 

37.00 

825 

The  motor  speeds  at  the  various  contact  points  being  assumed  as 
above,  the  c.e.m.f.  corresponding  to  these  speeds  can  be  readily 
determined  by  substituting  them  in  equation  (7),  p.  21.  Knowing 


SHUNT-MOTOR  STARTING   BOXES. 


35 


the  c.e.m.f.  and  taking  the  corresponding  values  of  armature  drop, 
also  the  brush  contact  drop  from  the  curve  in  Fig.  2,  the  voltage  drop 
(IaRx)  in  the  external  resistance  Rx  can  be  calculated  from  the  fol- 
lowing equation  giving  the  relation  between  action  and  reaction  in 
the  armature  circuit  of  a  direct-current  shunt  motor: 


V  =  c.e.m.f. 


IaRa 


Db  +  IaRx. 


(9) 


TABLE  IV.— DERIVATION  OF  STARTING-BOX  RESISTANCES. 


>: 

a 

I 

.22 

t/r 

C.e.m.f.,  = 

o 

* 

PJ 

a 

1 

r.p.m.. 

Armature  Drop. 

2  cf 

H 

|'0? 

i 

1 

r.p.m.rc'e'm<  " 

/aff.. 

1 

<u 
H 

5 

g 

fi 

H 

w 

w 

Volts. 

Volts. 

Volts. 

Volts. 

Ohms. 

230 

-£-  218.2  =  0 

55.5    X  .28     =  15.6 

1.71 

212.7 

3.83 

1 

825 

230 

|^  218.2  =  119.0 

49.3   X  .28    =  13.8 

1.60 

95.6 

1.94 

2 

825 

640 

230 

7  218.2=  169.0 

44.5    X  .28    =  12.2 

1.50 

47.3 

1.06 

3 

825 

750 

230 

—  218.2  =  198.5 

41.7    X  .28    =  11.7 

1.45 

18.35 

.44 

4 

825 

800 

230 

|^  218.2=  212.0 

39.5    X  .28    =  11.1 

1.43 

5.50 

.14 

5 

825 

820 

230 

rrr  218.2  =  217.0 

37.75X  .28    =  10.6 

1.41 

1.00 

.027 

9 

825 

C0f> 

230 

2=  218.2=  218.2 

37.      X  .281  =  10.4 

1.49 

.0 

.0 

7 

825 

Motor  starting  boxes  are  not  designed  with  large  radiating  sur- 
faces, because  the  box  is  supposed  to  be  in  use  only  for  very  short 
periods  (15  to  30  seconds  at  one  time).  The  chief  precaution  is  to 
have  the  resistance  units  of  such  current-carrying  capacity  that  they 
do  not  become  injured  by  a  momentary  overload  current  of  50  per 
cent  above  the  rated  value.  Practically  the  box  should  be  designed 
to  act  a  certain  number  of  times  per  hour  (every  4  minutes)  with- 
out injury.  If  the  contact  arm  of  the  starting  box  is  moved  too 
rapidly  over  the  contact  points,  the  armature  current  would  be 
excessive  and  injury  might  result,  hence  the  starting  period  of  15  to 
30  seconds  as  stated.* 

*  Standardization  Rules  A.  I.  E.  E.,  Division  II,  Section  H,  Rule  302. 


36        ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

In  case  a  motor  is  to  be  started  without  load,  the  starting  resist- 
ances calculated  before  would  not  bring  up  the  speed  gradually, 
because  the  initial  IfaRx  drop  would  be  only  3.83  X  2.3  or  8.8  volts, 
so  that  the  c.e.m.f.  with  total  box  resistance  in  circuit  and  no-load 
current  is  230  —  8.8  +  1.4  =  219.8  volts,  and  the  corresponding 
motor  speed  would  be  (219.8  -5-  218.2)  825  =  832  r.p.m.  Hence 
the  motor  would  almost  instantly  run  at  a  speed  greater  than  at  rated 
load.  For  this  reason,  to  start  a  motor  without  load  it  is  sometimes 
the  practice,  especially  in  Europe,  to  use  a  special  box,  the  resistance 
of  which  is  much  higher  than  in  the  preceding  design.  For  instance, 
if  it  were  desired  to  have  the  "speeding  up"  curve  at  no  load  of  the 
same  shape  and  corresponding  values  as  that  in  Fig.  3,  the  required 
external  resistance  would  be  far  higher  than  those  in  Table  IV.  For 
example,  the  armature  current  running  free  is  only  2.3  amperes, 
which  with  the  armature  "cold"  (25  degrees  C.)  would  produce  an 
armature  drop  of  2.3  X  .24  =  .55  volt.  This  added  to  the  no-load 
brush  drop  of  .80  volt  makes  a  total  drop  of  only  1.35  volts,  so  that 
the  drop  in  the  starting  box  would  have  to  be  230  —  1.4  =  228.6 
volts.  Therefore  the  required  starting  resistance  Rx  =  228.6  •*•  2.3 
=  99.4  ohms  in  order  that  the  armature  should  stand  still  and  carry 
only  2.3  amperes.  If  a  starting  box  having  any  such  high  resistance 
were  employed  to  start  a  motor  under  load  (for  example  at  rated 
torque) ,  the  armature  current  would  be  far  less  than  the  rated  value 
until  next  to  the  last  contact  point  was  passed,  when  the  armature 
would  suddenly  jump  to  rated  speed,  a  change  much  too  sudden  and 
likely  to  injure  the  motor  or  the  machine  driven  by  it.  For  this 
reason,  and  to  have  the  starting  box  adapted  to  all  load  conditions, 
the  American  practice  is  to  use  a  box  designed  to  start  the  motor 
under  load.  Thus,  while  the  speed  may  rise  rather  suddenly  with- 
out load,  no  injury  will  result,  because  the  initial  (i.e.,  total)  resistance 
is  sufficient  to  limit  the  armature  current  to  a  reasonable  value  and 
the  speed  does  not  rise  more  than  about  3  per  cent  above  rated  value. 
Rapid  acceleration  without  load  is  not  objectionable,  provided 
neither  current  nor  speed  is  excessive.  On  the  other  hand,  the  motor 
is  fully  protected  when  started  under  load,  the  armature  current 
having  sufficient  strength  to  bring  the  armature  gradually  up  to 
speed. 

Starting-Box*  Connections. —  Care  should  be  taken  with  regard 
to  the  disconnection  of  shunt  motors  from  the  source  of  supply  in 


SHUNT-MOTOR  STARTING   BOXES.  37 

order  that  the  field  circuit  shall. not  be  broken  suddenly  when  the 
motor  is  shut  down.  Failure  to  take  this  precaution  not  only  causes 
arcing  at  the  switch  blades,  but  might  break  down  the  insulation  of 
the  field  coils,  owing  to  the  high  voltage  induced  by  sudden  cessation 
of  current  in  such  an  extremely  inductive  winding.  The  motor  can 
be  disconnected  sparklessly  and  without  danger  to  the  field  insulation 
by  opening  the  supply  switch  at  S,  leaving  the  field  connected  across 
the  armature,  Fig.  i.  The  machine  then  slows  down  gradually  and, 
acting  as  a  generator,  sends  a  decreasing  current,  in  the  original 
direction,  through  the  field  winding,  so  that  a  sudden  inductive  dis- 
charge is  avoided. 

To  avoid  the  possibility  of  closing  the  supply  switch  with  the 
starting  resistance  cut  out,  the  National  Board  of  Fire  Underwriters 
specify  that  ''Motor-starting  rheostats  must  be  so  designed  that 
the  contact  arm  cannot  be  left  on  intermediate  segments,  and  must 
be  provided  with  an  automatic  device  which  will  interrupt  the  supply 
circuit  before  the  motor  speed  falls  to  less  than  one-third  of  its 
normal  value."  *  This  protective  feature  consists  in  replacing  the 
starting  resistance  in  the  armature  circuit,  upon  opening  of  the 
supply  switch,  and  is  automatically  accomplished  by  an  auxiliary 
device  in  the  starting  box.  This  comprises  a  small  electro-magnet  B, 
excited  by  the  supply  voltage,  so  placed  as  to  hold  the  contact  arm, 
through  a  keeper  K  placed  upon  it,  in  the  "on"  position  as  long  as 
the  circuit  is  closed.  After  the  supply  circuit  is  broken  the  contact 
arm  is  returned  to  the  "off"  position  by  means  of  a  spring  at  P,  but 
this  does  not  occur  until  the  speed  of  the  motor  has  been  consider- 
ably reduced.  The  winding  of  the  electro-magnet  B  may  be  placed 
in  series  with  the  shunt  field  or  directly  across  the  line,  the  latter 
connection  being  preferable  with  adjustable-speed  motors. 

*  Rule  60,  section/,  National  Electric  Code,  1907. 


STARTERS  AND  REGULATORS.    R.  Krause.    London,  1904. 

RHEOSTAT  CONTROL.     A.  C.  Button.     Gen.  Elect.  Review,  Schenectady,  Vol.  XII, 
1909,  pp.  365,  423. 


CHAPTER   V. 

SHUNT-MOTOR    SPEED   CONTROL  BY  VARIATION  OF 
ARMATURE  RESISTANCE. 

THE  service  conditions  under  which  electric  motors  operate  often 
require  adjustable  speeds  which  are  under  the  control  of  the  oper- 
ator. This  adjustment  may  be  accomplished  in  various  ways,  and 
the  particular  method  to  employ  depends  upon  the  character  of 
work,  range  of  speed  required,  cost  of  electrical  energy  as  well 
as  cost  of  motor  and  equipment.  There  are  two  general  condi- 
tions of  speed  control;  the  first  calling  for  adjustable  speeds  at  con- 
stant torque  (if  desired),  the  second  being  satisfied  by  adjustable 
speed  with  variable  torque.  The  first  of  these  conditions  is  fulfilled 
by  variation  of  the  applied  armature  voltage,  and  the  second  by 
change  of  field  flux. 

i.  Armature  Rheostat  Control.  The  first  method  which  natu- 
rally suggests  itself  is  the  variation  of  armature  voltage  by  means  of 
resistance  in  series  with  it.  The  current-carrying  capacity  of  this 
regulating  resistance  must  naturally  be  greater  than  that  of  the 
ordinary  starting  box,  since  it  may  be  in  circuit  for  long  periods. 
The  accepted  design  is  such  that  it  will  not  heat  up  to  more  than 
100  degrees  C.  on  continuous  service  with  rated  load  current.  The 
large  current  capacity  may  be  obtained  by  making  up  the  resistance 
units  in  plate  form,  or  some  other  arrangement  by  which  large 
radiating  surface  is  obtained.  Consideration  of  equation  9, 

V=  c.e.m.f.  +  IaRa  +  Db  +  IaRx,         (9) 

shows  that  with  increase  of  voltage  drop  in  the  rheostat  the  c.e.m.f. 
must  decrease,  and,  as  already  shown,  when  c.e.m.f.  is  reduced  the 
speed  diminishes  in  the  same  ratio,  hence  the  control  of  a  motor  by 
rheostat  in  its  armature  circuit  is  a  method  of  speed  regulation 
which  can  only  decrease  the  speed  of  the  machine. 

Speed-control  Rheostat.  — For  the  sake  of  simplicity  in  the  fol- 
lowing problems,  the  stray-power  losses  will  be  assumed  to  be  the 

38 


SHUNT-MOTOR  SPEED   CONTROL. 


39 


same  at  no  load  and  full  load  — in  fact,  as  already  shown,  the  change 
affects  the  efficiency  by  only  a  very  small  amount.  Assume  that  it 
is  required  to  design  a  resistance  box  to  be  employed  as  a  speed 
controller  for  the  lo-h.p.  motor,  and  this  control  is  to  give  a 
speed  variation  in  four  steps  from  one-quarter  to  rated  speed,  at 
rated  torque  and  for  continuous  service  at  any  of  the  four  speeds. 
Since  rated  torque  is  to  be  developed,  the  motor  armature  should  be 
considered  as  operating  "hot,"  hence  its  resistance  Ra  is  .28  ohm 
and  the  speeds  desired  are  206,  412,  619,  and  82 5  r.p.m.,  respectively, 
the  last  being  the  rated  speed.  Tabulating  the  conditions  of  torque 
and  speed  desired  and  recollecting  that  IaRx  =  V  -  (c.e.m.f. 
+  IaRa  +  Db)  we  obtain  the  following  results: 


TABLE  V.  — DERIVATION  OF  CONTROLLER-BOX  RESISTANCES. 


A 

B 

N 

Speed 
Desired, 

c.e.m.f.  = 

Armature 
Drop 

Brush   Drop 

External  Drop 
=  230  - 

(A  +  B-f  N) 

External 
Resist. 
R. 

r.p.m. 

!     P.  Ill  'X   ctt  O    O 

/.R. 

Db  Volts. 

=  /./?. 

Ohms. 

r  p.m.r 

37X.28  Volts. 

Volts. 

Volts 

i          =  206 

54.55 

10.4 

1.4 

163.6 

4.4 

=  412 

109.10 

10.4 

1.4 

109.1 

2.95 

|          =  619 

163.60 

10.4 

1.4 

55.6 

1.48 

rated  =  825 

218.20 

10.4 

1.4 

0.0 

0. 

The  same  results  are  obtained  very  simply  as  follows:  The  c.e.m.f. 
at  rated  speed  and  torque  being  218.2  volts  (=  230  —  n.8),  the 
armature  would  stand  still  if  sufficient  resistance  were  put  in  series 
with  it  to  produce  a  drop  of  this  amount, "that  is,  IaRx  =  218.2  volts. 
The  rated  current  Ia  is  37  amperes,  therefore  the  required  resistance 
Rx  =  218.2  -r-  37  =  5.9  ohms.  Since  this  external  resistance  gives 
zero  speed  with  rated  torque  exerted,  three-quarters  of  5.9,  or  4.4, 
ohms  give  one-quarter  speed,  also  5.9  H-  2  =  2.95  ohms  give  one- 
half  speed,  5.9  -r-  4  =  1.48  ohms  give  three-quarters  speed  and  so 
on.  With  armature  current  raised  to  1.25  /^corresponding  to  stand- 
ard overload  capacity,  the  necessary  resistance  in  each  case  is  only 
.8  as  great.  In  this  way  the  resistance  needed  for  any  speed  from 
zero  to  rated  value  and  for  any  armature  current  can  readily  be 
determined. 


40        ELECTRIC  MOTORS,    THEIR   ACTION  AND   CONTROL. 

Knowing  the  value  in  ohms  of  the  various  resistances  required  in 
the  controller,  it  is  still  necessary  to  determine  the  size  of  box  that 
will  contain  the  needed  radiating  surface.  The  temperature  rise 
usually  permitted  in  speed-regulating  rheostats  is  100  degrees  C., 
and  with  temperature  limit  known,  the  surface  of  the  resistance 
units  (^4)  can  be  calculated,  because  the  heat  energy  emitted  must 
equal  that  produced;  that  is, 

.24I\Rx=Aht,  (10) 

in  which  Ia  is  the  rated  current,  Rx  the  total  resistance  of  the 
rheostat,  h  the  emissivity  of  the  resistance  metal  (i.e.,  gram-calories 
emitted  per  square  centimeter  per  degree  C.),  and  /  the  difference 
between  room  and  rheotsat  temperatures  in  degrees  C.  The  total 
controller  resistance  Rx  as  given  in  preceding  table  is  4.4  ohms,  the 
current  7ft  is  37  amperes,  the  emissivity  h  of  nickelin  (the  resistance 
metal  assumed)  is  .000506  and  the  difference  in  temperature  t  is  100 
degrees  C.  Substituting  these  values  in  equation  (10)  we  have 

-24  X      2X4-4  or    28,600  sq.  cm. 


.000506  X  ioo 

That  is,  the  nickelin  wire  employed  must  have  an  emitting  surface 
of  28,600  sq.  cm.  in  order  that  it  will  not  increase  in  temperature 
more  than  ioo  degrees  C.  at  rated  load.  Knowing  the  resistance 
required  and  the  radiating  surface,  the  length  of  the  resistance 
wire  can  readily  be  calculated  if  the  cross  section  be  decided  upon. 
Assuming  a  circular  wire,  the  following  formulas  of  resistance  and 

pi 
length  apply:  Surface  area  =  2  nlr  and  Rr  =  —  ,  where  area  =  28,  600 

sq.  cm.,  I  the  length  in  centimeters,  r  the  radius  in  centimeters,  Rr 
the  total  rheostat  resistance  (4.4  ohms)  and  p  the  specific  resistance 
of  nickelin  (i.e.,  .00005  °^m  Per  centimeter  cube).  Substituting 
these  values  in  the  above  equations  we  have 

28,600  /  4-4  ^2 

28,600  =  2  nlr  or  /  =  -  :  and  4.4  =  .00005  —  7  or  £  =  —    —  • 
2  TIT  Tzr2  .00005 

Equating  these  values  of  I  and  solving  for  r,  we  obtain  r  =  .253  cm.; 
from  which  /  =  18,000  cm.  That  is,  the  nickelin  wire  required 
would  have  a  diameter  of  .506  cm.  (No.  4  B.  &  S.  gauge)  and  a 
length  of  1  80  meters.  The  total  resistance  is  divided  into  three 


SHUNT-MOTOR  SPEED   CONTROL. 


41 


sections  of  1.47  ohms  each  to  give  three-quarters,  one-half  and 
one-quarter  speed  respectively  when  the  sections  are  successively 
introduced  into  the  circuit. 

A  simple  method  of  constructing  this  box  is  to  employ  resistance 
units  in  the  form  of  helices.  A  helix  4  cm.  diameter  requires  TT  4  or 
12.5  cm.  of  wire  per  turn,  or  i  meter  of  wire  would  make  eight  turns. 
These  turns  should  be  about  .5  cm.  apart,  which  is  equal  to  the 
diameter  of  the  wire,  so  that  eight  turns  require  8  cm.  height.  The 
total  length  of  resistance  wire  being  180  meters,  each  section  is  60 
meters  long  and  is  composed  of  60  X  8  or  480  turns.  Hence  each 
section  if  made  up  in  one  helix  would  be  480  cm.  high.  In  order 
that  a  section  should  not  be  too  long,  it  may  be  divided  into  12  units, 
each  40  cm.  high.  For  the  complete  box  36  units  are  required,  each 
40  cm.  high  and  4  cm.  in  diameter.  These  coils  should  be  placed  so 
as  to  have  i  cm.  space  between  them  for  ventilation  and  to  avoid 
short-circuiting;  hence  the  box  would  be  6  X  5  or  30  cm.  wide  and  of 
the  same  thickness.  The  inside  dimensions  of  the  finished  box 
would  accordingly  be  40  X  30  X  30  cm.,  containing  180  meters  of 
nickelin  resistance  wire  No.  4  gauge. 

Discussion  of  Speed  Control  by  Armature  Rheostat.  —  The  effici- 
ency of  such  a  combination  for  obtaining  motor  speed  control  at 
the  various  r.p.m.  selected  is  determined  as  follows:  The  ^tray- 
power  losses  affect  the  efficiency  at  the  various  speeds  so  slightly 
that  for  this  calculation  we  assume  them  as  constant  at  their  value 
for  rated  torque  and  speed;  correction  can,  however,  be  made,  as 
shown  later. 

A  motor  operating  at  a  constant  torque  will  have  an  output 
directly  proportional  to  its  r.p.m.;  hence,  we  have  the  following 
tabulated  results  for  the  typical  lo-horsepower  machine  the  actual 
output  of  which  is  7550  watts. 


TABLE  VI.— EFFICIENCY  OF  SPEED  CONTROL  BY  ARMATURE 
RHEOSTAT  AT  RATED  TORQUE. 


Total  Watts  Input. 

R.p.m. 

Watts  Output. 

Efficiency  . 
Per  Cent 

220  X  37  +  230  X  1  =  8740 

825 

7550 

86.5 

220  X  37  +  230  X  1  =  8740 

619 

5662 

64.8 

220  X  37  +  230  X  1  =  8740 

412 

1887.5 

43.2 

220  X  37  +  230  X  1  =  8740 

206 

3775 

21.6 

42        ELECTRIC   MOTORS,    THEIR   ACTION   AND   CONTROL. 

If  we  plot  these  efficiency  values  as  ordinates  and  the  speeds  as 
abscissas,  we  have  a  straight  line,  as  in  Fig.  4.  Hence  it  is  sufficient 
to  calculate  one  value  and  draw  a  line  through  this  point  and  the 
origin.  A  study  of  this  method  of  speed  control  brings  out  the 
following  objections: 


% 

.p.m. 

/ 

/ 

/ 

/ 

' 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

2 

8 

4 

I 

6 

9 

825 

Percent  of  Rated  Speed 

FIG.    4.  —  EFFICIENCY  CURVE  OF   IO-HORSEPOWER   MOTOR    WITH    ARMATURE 

RHEOSTAT    CONTROL 

Objections  to  Armature  Rheostat  Control. 

(a)  Bulk  of  Rheostat.  —  This  may  not  be  very  objectionable  if 
only  a  few  motors  are  so  controlled,  but  for  a  number,  the  extra 
space  becomes  a  factor,  and  in  many  cases  it  is  difficult  to  find 
sufficient  room  near  the  motor. 

(b)  Inefficiency  of  the  System.  —  The  same  amount  of  power  is 
supplied  at  all  speeds,  but  at  low  speeds  only  a  small  part  of  it  is 
converted  into  useful  work,  the  balance  being  wasted  as  heat.     Thus, 
with  the  zo-horsepower  motor  the  useful  work  at  one-quarter  speed 
is  only  21.4  per  cent  of  the  total  input,  as  shown  in  table  above. 

(c)  Poor  Speed  Regulation  with    Varying    Loads.  — Since    the 
impressed  voltage  at  the  armature  terminals  is  equal  to  the  line 
voltage  minus  the  resistance  drop  in  the  regulator  (Vt  =  V  —  IaRx), 
any  change  in  the  current  drawn  by  the  motor  produces  a  change 
in  the  terminal  voltage,  the  c.e.m.f.,   and,  therefore,  the  speed. 
The  changes  in  speed  likely  to  occur  with  load  changes  may  be 
very  great,  and  are  best  brought  out  by  specific  examples.     Consider 


SHUNT-MOTOR  SPEED   CONTROL.  43 

the  io-h.p.  motor  with  the  controller  box  just  designed  for  it,  and 
assume  that  the  motor  is  driving  a  lathe  at  rated  torque  (37  amperes) 
and  at  one-half  speed.  That  is,  the  motor  is  running  at  412  r.p.m. 
and  a  full  cut  is  being  made;  suddenly,  however,  the  machinist  so 
changes  the  depth  of  cut  that  the  torque  falls  to  one-quarter  of  the 
rated  value  — that  is,  70  =  9.25  amperes —  and  the  simultaneous 
increase  in  speed  accompanying  this  will  be  very  considerable.  Its 
value  can  be  determined  as  follows: 

The  total  resistance  drop  in  the  armature  circuit  is  at  this  reduced 
current  equal  to  Ia  (Ra  +  Rr)  +  Db  =  9.25  (.28  +  2.9)  +  i  = 
30.4  volts,  so  that  the  c.e.m.f.  =  230  —  30.4  =  199.6  volts.  Hence 

the  speed  =  -      -  X  825  =  755  r.p.m.,  that  is,  it  rises  suddenly  from 
218.2 

412  to  755  r.p.m.  when  the  torque  is  decreased  from  rated  value  to 
one-quarter  thereof.  In  most  practical  cases  it  is  desirable  that  no 
material  variation  in  speed  should  occur  with  change  of  torque,  so 

that  this  increase,  amounting  to  83   per  cent  ( —   =1.83),  would 

\4I2  / 

usually  be  very  objectionable. 

The  speed  change  for  any  torque  variation  at  any  rheostat  setting 
can  be  similarly  calculated  and  the  results  of  such  calculation  have 
been  embodied  in  the  speed-regulation  curves  of  Fig.  5.  The  speed 
of  the  motor  at  any  fraction  of  rated  torque  with  any  position  of  the 
controller  arm  can  be  obtained  from  these  curves  and  the  speed 
change  due  to  any  variation  of  torque  thus  readily  determined. 
For  example,  the  typical  lo-horsepower  motor,  when  operated  at 
rated  torque  with  3  ohms  external  resistance  in  its  armature  circuit, 
would  have  a  speed  of  about  412  r.p.m.  or  half  the  rated  value, 
according  to  Fig.  5.  If  its  load  be  so  changed  that  the  required 
torque  falls  to  one-half  of  the  previously  assumed  value,  the  regula- 
tion curve  indicates  that  the  speed  rises  to  635  r.p.m.,  an  increase 
of  54  per  cent.  From  the  preceding  examples  it  becomes  obvious 
that  the  speed  regulation  of  a  motor  with  armature  rheostat  is  very 
poor;  in  fact  the  changes  in  speed  which  occur  with  considerable 
variations  in  torque  are  so  excessive  as  to  be  very  objectionable  and 
may  actually  endanger  the  tool  and  its  work.  This  difficulty  of 
poor  speed  regulation  with  variation  of  torque  may  be  avoided  by 
employing  some  of  the  field-weakening  or  multiple-voltage  methods 
to  be  considered  later.  With  these  methods,  even  when  the  torque 


44        ELECTRIC   MOTORS,    THEIR  ACTION  AND   CONTROL. 

varies  from  zero  to  rated  value,  the  speed  change  is  small,  being  from 
•2  to  8  per  cent,  depending  upon  the  particular  method  employed. 

In  the  foregoing  calculations,  the  error  introduced  by  assuming 
the  stray-power  losses  as  constant  at  all  speeds  is  small,  as  shown  in 
the  following  paragraph: 

Consider,  for  example,  the  one-quarter  speed  condition  for  which 
the  error  introduced  by  variation  of  stray-power  losses  due  to 
speed  change  would  be  a  maximum.  With  speed  at  one-quarter  of 
rated  value,  and  at  rated  torque,  the  motor  output  would  be  equal  to 
1887.0  watts.  The  various  resistance  losses  would  be  the  same  as  at 


Percent  of  Rated  Torque 

FIG.    5.  —  SPEED    CURVES    OF    IO-H.P.    MOTOR,    WITH    DIFFERENT    ARMATURE 
RHEOSTAT    RESISTANCES. 

rated  load,  or  675.0  watts.  The  total  stray-power  loss  is  made  up  of 
eddy  current,  hysteresis,  friction  and  windage  losses,  the  first  of 
which  varies  as  the  square  of  the  speed,  while  the  others  vary  directly 
with  speed.  In  motors  of  about  ic-h.p.  capacity  the  various  losses 
are  approximately  in  the  following  proportion: 

Eddy-current  loss,  one-quarter  of  total  stray-power  loss. 

Hysteresis  loss,  one-quarter  of  total  stray-power  loss. 

Friction  and  windage,  one-half  of  total  stray-power  loss. 

The  sum  of  these  losses,  or  the  total  stray  power,  has  already 
been  found  to  be  526  watts  (p.  24).  Hence  the  eddy-current  loss 
at  normal  speed  is  13 1.5  watts,  and  at  one-quarter  speed  is  13 1.5  -v-4"2 
=  8.2  watts.  The  hysteresis  loss  at  normal  speed  is  131.5,  and  at 


SHUNT-MOTOR  SPEED   CONTROL. 


one-quarter  speed  is  131.5  -f-  4  =  32.8  watts.  The  friction  and  wind- 
age loss  at  normal  speed  is  263  watts,  and  at  one-quarter  speed  is 
263-^4=  65.6  watts. 

Adding  these  values,  the  corrected  stray-power  losses  for  one- 
quarter  speed  are  107  watts.  The  total  motor  losses  at  this  speed 
are,  therefore,  675  +  107  =  782  watts.  The  motor  input  is  8740 
watts;  motor  loss,  782  watts,  and  motor  output  is  1887  watts; 
hence,  the  loss  in  the  rheostat  is  8740-  (1887  +  782)  =  6071  watts; 
thus  Rx  =6o7i^372  =4.45  ohms,  or  only  .05  more  than  that 
obtained  by  calculation  with  stray-power  losses  assumed  constant. 
Hence  the  error  introduced  by  this  assumption  is  negligible. 

2.  Speed  Control  by  Brush  Shifting.  —  Speed  variation  by  means 
of  brush  shifting  is  not  desirable,  as  it  increases  armature  reaction 
and  tends  to  produce  sparking. 


illO 


At  rated  Load  motor  has  4.5  %  la  R*  Drop 

a    Brushes  in  minimum  sparking  position 

b    Brushes  lagging  a  trifle 

C    Brushes  lagging  considerably 

d    Brushes  leading  a  trifle 


25  50  75  100 

Percent  Rated  Torque 

FIG.  6. RELATION  BETWEEN  BRUSH  POSITION  AND  SPEED  OF  SHUNT  MOTOR. 

Curves  are  given  in  Fig.  6  showing  how  the  speed  can  be  controlled 
by  brush  shifting,  but  the  range  is  small.  It  is  interesting  to  note  in 
connection  with  this  figure  that  armature  reaction  in  a  motor  due  to  a 
considerable  negative  lead  of  the  brushes  (that  is,  moved  backwards, 
opposite  to  direction  of  rotation)  can  be  made  to  maintain  a  constant 
or  rising  speed.  This  is  not  a  feasible  method,  however,  because 
in  the  operation  of  direct-current  motors  it  is  practically  necessary 
to  set  the  brushes  in  the  position  of  minimum  sparking,  since  spark- 
ing is  the  most  troublesome  feature  of  these  machines.  In  the  case 
of  interpolar  or  commutating  pole  motors  a  lag  of  even  ^  to  7V  inch 
in  brush  position  causes  a  pronounced  rise  in  speed  as  the  load  is 
increased,  with  the  probable  production  of  visible  sparking  (p.  64) . 


CHAPTER   VI. 

SPEED    CONTROL   OF   SHUNT   MOTORS    BY  VARIATION 
OF   FIELD    CURRENT. 

IN  the  preceding  chapter,  the  speed  regulation  of  a  motor  by 
means  of  an  adjustable  resistance  in  the  armature  circuit  was  con- 
sidered, and  the  objections  to  its  use  pointed  out.  This  chapter  is 
devoted  to  the  discussion  of  a  second  method  of  speed  adjustment 
not  open  to  the  same  objections. 


2  p 

The  equation  e  =  -  shows  that  if  e,  the  counter  e.m.f., 
60  X  io8  X  b 

is  kept  constant,  and  <£  varied,  the  speed  N  varies  inversely  as  the 
flux  $  because  the  other  quantities  do  not  change  unless  purposely 
made  to  do  so  by  altered  construction  or  arrangement  of  parts. 
This  relation,  therefore,  indicates  a  method  of  speed  variation. 
Shunt  motors  are  usually  designed  to  have  such  high  flux  density 
in  both  field  and  armature  that  it  is  not  practicable  to  increase  it 
materially.  Hence  this  method  is  confined  to  and  commonly  called 
field  weakening. 

In  the  case  of  ordinary  shunt  motors,  the  range  of  speed  variation 
by  means  of  field  weakening  is  small.  For  instance,  take  the  10- 
horsepower  motor  previously  considered  and  weaken  its  field  by  the 
introduction  of  extra  resistance  into  its  field  circuit  to  produce  30  per 
cent  increase  in  speed.  Since  the  flux  must  be  varied  inversely  as 
the  speed,  it  must  be  weakened  in  the  proportion  130  :  100  or 
100  :  76.9,  that  is,  23.1  per  cent,  while  to  produce  this  change  the  field 
ampere-turns  must  be  reduced  by  about  50  per  cent.  To  develop 
rated  torque  with  this  diminished  field  strength  the  armature  current 
must  be  increased  30  per  cent,  and  the  ratio  between  back  ampere- 
turns  and  field  armature-turns,  instead  of  having  its  normal  value  of 

about  .10,  is  raised  to  —  =  .26,  because  the  former  is  30  per  cent 

greater  and  the  latter  is  reduced  to  one-half.  This  latter  ratio  is 
excessive.  The  corresponding  increase  in  cross  ampere-turns,  acting 
collectively  with  the  increased  back  ampere-turns,  causes  excessive 

46 


SPEED    CONTROL  OF  SHUNT  MOTORS.  47 

sparking.  Hence  a  30  per  cent  increase  of  speed  with  an  ordinary 
standard  shunt  motor  cannot  be  obtained  without  objectionable 
sparking. 

Another  difficulty  arises  from  the  fact  that  the  increase  of  armature 
current  necessary  to  maintain  constant  torque  augments  the  PaRa 
loss,  which  in  the  lo-horsepower  motor  armature  rises  from  372  X 
.28  to  48*  X  .28,  an  increase  of  262  watts  or  68  per  cent,  producing 
too  much  heat  for  the  armature  insulation  to  stand  for  any  consider- 
able time. 

Adjustable  speed  motors  of  the  flux-variation  type  are  not  con- 
stant-torque machines,  but  constant-horsepower  or  output  motors;  i.e., 
the  torque  falls  as  the  speed  increases  in  inverse  ratio,  or  T  X  r.p.m. 
=  a  constant.  In  fact,  unless  the  ratio  of  back  ampere-turns  to 
field  ampere-turns  is  less  than  10  per  cent  at  minimum  speed,  an 
increase  in  speed  of  even  30  per  cent  with  constant  output  is  not 
practicable  with  the  ordinary  shunt  motor  because  it  demands  a 
50  per  cent  reduction  in  field  m.m.f.,  as  shown  above. 

It  is  evident  that  a  shunt  motor,  to  have  any  considerable  range  of 
speed  variation  (i.e.,  increase  of  more  than  20  or  30  per  cent)  by 
field  weakening,  requires  some  modification  in  design,  because  the 
field  must  be  more  powerful  with  respect  to  the  armature  than  in  the 
case  of  standard  single-speed  motors.  Some  special  motors  of  this 
kind  allow  of  speed  variations  of  three  or  four  to  one,  with  constant- 
horsepower  output,  but  not  at  rated  torque.  These  increased  speed 
ranges  are  obtained  as  follows: 

(a)  Magnetic  Circuit  of  Very  Soft  Steel. — The  magnetic  properties 
of  the  material  are  such  that  even  with  high  flux  densities  the  bend 
of  the  curve  is  not  reached,  so  that  the  change  in  field  ampere-turns 
to  produce  a  large  change  in  flux  is  not  excessive;  i.e.,  the  rate  of 
change  of  flux  and  m.m.f.  is  almost  in  direct  proportion.  With 
these  machines  the  field  frame  is  also  large,  the  total  flux  being  very 
great,  while  the  armature  winding  consists  of  fewer  turns  per  section 
and  a  larger  number  of  sections,  so  that  the  self-induction  per  section 
is  low.  Thus,  under  normal  or  even  exceptional  conditions  the 
ratio  of  field  to  back  ampere-turns  is  kept  within  10  per  cent.  How- 
ever, a  machine  having  a  field  of  sufficient  strength  to  prevent  spark- 
ing at  high  speeds  must  have  a  frame  considerably  larger  than  neces- 
sary for  a  single-speed  machine  of  equal  power  and  the  commutator 
must  have  a  greater  number  of  bars.  In  general,  when  considering 


48        ELECTRIC  MOTORS,    THEIR   ACTION   AND   CONTROL. 

this  simple  type  of  variable-speed  motor,  it  can  be  stated  that  the 
percentage  of  speed  increase  of  which  a  normally  loaded  motor  is 
capable  by  means  of  field  weakening  is  a  measure  of  its  overload 
capacity  with  full  field  strength.  In  other  words,  a  given  range  of 
speed  variation  demands  a  motor  having  a  certain  increased  capac- 
ity, or  special  features  of  design.  The  following  practical  examples 
illustrate  this  point: 

Relative  Sizes  of  Frames  -with  Speed  Ratio  o)  i  :  2: 

3 -horsepower  frame  for  a  2-horsepower  motor. 

1 5 -horsepower  frame  for  a  lo-horsepower  motor. 

Relative  Sizes  of  Frames  with  Speed  Ratio  0/1:3: 

3-horsepower  frame  for  a  ij-horsepower  motor. 

lo-horsepower  frame  for  a  5 -horse power  motor. 

2o-horsepower  frame  for  a  lo-horse power  motor. 

(b)  With  the  magnetic  circuit  specially  designed  so  that  the  flux 
density  is  always  great  at  the  pole  tips,  the  field  distortion  due  to 
armature  reaction  is  lessened  and  a  sufficient  flux  is  maintained  in 
the  commutation  zone,  giving  sparkless  operation  within  reasonable 
speed  and  load  limits. 

Greater  ranges  of  speed  adjustment  than  a  three  to  one  ratio  are 
frequently  required,  for  example,  five  or  even  six  to  one;  in  such 
cases  the  preceding  types  are  not  economically  available.  With  the 
(a)  and  (b)  types  it  is  difficult  or  costly  to  maintain  the  commutation 
fringe  when  the  main  field  excitation  is  reduced  sufficiently  to  obtain 
a  speed  range  greater  than  four  to  one;  thus  some  new  feature  in 
design  to  maintain  the  commutation  flux  becomes  necessary.  This 
feature,  somewhat  differently  obtained,  is  present  in  two  types  of 
motors. 

(c)  Historically  the  first  of  these  is  the  Thompson-Ryan  design  of 
compensated  motor,  manufactured  by  the  Ridgway  Dynamo   and 
Engine  Works.     This  compensated  form  employs  what  is  equiva- 
lent to  a  stationary  armature  built  up  in  the  polar  faces,  and  traversed 
by  the  armature  current  or  a  portion  thereof,  which  develops  a 
m.m.f.  opposed  to  the  armature  m.m.f.,  thus  eliminating  or  even 
reversing  armature  reaction.     It  has  been  found,  however,  by  M.  E. 
Thompson  that  this  compensating  winding,  by  itself,  is  not  sufficient 
to  prevent  sparking  at  the  brushes,  so  he  introduced  commutating 
lugs,, excited  by  special  turns  immediately  around  them  as  well  as  by 
the  compensating  winding,  which  establish  a  flux  for  reversal  over 


SPEED   CONTROL   OF  SHUNT   MOTORS.  49 

the  coils  undergoing  commutation.*  This  motor  thus  possesses  the 
two  features  of  compensation  and  commutation,  which  are  independent 
of  the  strength  of  the  main  field,  and  sparkless  operation  over  wide 
speed  changes  is  theoretically  possible. 

(d)  The  second  of  these  special  forms  is  one  wherein  armature 
reaction  and  distortional  effects  are  not  overcome,  but  their  presence 
is  depended  upon  to  obtain  good  speed  regulation.  The  sparkless 
condition  of  operation  is  secured  by  the  use  of  "interpoles"  or 
auxiliary  field  poles,  placed  directly  over  the  zone  of  commutation, 
the  m.m.f.  of  these  poles  being  opposed  to  that  of  the  armature,  and, 
as  they  are  energized  by  coils  carrying  the  armature  current,  their 
m.m.f.  increases  with  and  is  designed  to  be  superior  to  that  of  the 
armature.  Thus  the  flux  for  reversal  is  locally  maintained  inde- 
pendently of  the  main  field,  and  varies  automatically,  as  required, 
with  the  result  that  sparkless  commutation  may  be  obtained. f  The 
difference  between  types  c  and  d  is  that  the  former  embodies  general 
magnetic  compensation  as  well  as  local  commutation  flux,  whereas 
the  latter  depends  upon  local  flux  for  commutation  alone,  with  no 
attempt  to  neutralize  armature  reaction  as  a  whole. 

Machines  of  Class  (a)  are  built  by  many  manufacturers,  and  while 
a  number  are  in  use  they  are  larger  than  standard  constant- 
speed  motors,  as  already  shown.  An  example  of  this  class  (a) 
is  found  in  a  5-h.p.  Bullock  shunt  motor,  the  data  of  which  are  as 
follows: 

Rated  capacity,  5-h.p. 

Rated  pressure,  220  volts. 

Speed,  350  to  1050  r.p.m. 

Armature  current  at  rated  load,  22.2  to  24.6  amperes  (depending 
upon  the  speed). 

Field  current,  1.3  to  .23  amperes. 

No-load  armature  current,  2.1  to  3.6  amperes  (depending  upon 
field  strength  and  speed). 

Armature  resistance,  hot,  1.12  ohms. 

Field  resistance,  hot,  195  ohms. 

Weight  of  motor  complete,  noo  Ib. 

Tests  were  conducted  upon  this  motor  with  the  results  shown  in 
the  following  series  of  curves: 

*  U.  S.  Patent  No.  591,024,  October  5,  1897. 
t  U.  S.  Patent  No.  775,310,  November  22,  1904. 


50        ELECTRIC  MOTORS,    THEIR  ACTION   AND   CONTROL. 


zog 

175 
150 
126 
100 
75 
50 
25 
0 

J 

^ 

*~~~ 

// 

^ 

, 

// 

^ 

/I 

/ 

// 

K.p.m 

850 

// 

// 

.2           A           .6            .8          1.0          1.2        1.4 

Field  Current 

FIG.    7.  —  MAGNETIZATION   CURVE    OF    5-H.P.    BULLOCK   3  :  I    ADJUSTABLE- 
SPEED    SHUNT    MOTOR. 


1400 


1000 


COO 


Field  Current 


Current 


.23  Amp 


H.P.  Output 
FIG.   8.  —  SPEED-LOAD  CURVES  OF   5-H.P.    BULLOCK  MOTOR. 


SPEED   CONTROL   OF  SHUNT  MOTORS. 


51 


The  magnetization  curve  (Fig.  7)  of  this  motor  shows  no  special 
feature,  other  than  that  the  flux  density  is  kept  below  the  bend  of 
the  curve.  A  study  of  the  speed-load  curves  (Fig.  8)  shows  that 
the  speed  regulation  under  load  changes  is  reasonably  good  at  the 
lower  speeds.  At  the  highest  speed  (curve  A)  the  load  was  not 
carried  beyond  5  horsepower,  the  rated  value,  the  reason  being  that 
at  this  point  the  tendency  to  spark  at  the  brushes  becomes  pro- 
nounced and  the  speed  regulation  not  as  good  as  in  the  preceding 
speed-load  curves.  The  larger  falling  off  in  speed  in  this  case  was 
due  to  the  greater  sparking  and  voltage  drop  at  the  brush  and  com- 
mutator contacts.  The  currents,  running  free,  rise  with  increase  of 
speed,  owing  to  greater  core  and  friction  losses. 


Curre  A  No  load  R.p.m.410  AmuCurremt  4  Amps.      |_  Field  Current 
X      Curve  B  Rated  load  R.p.m.350  Arm.Current  25  Amps.  \   —  1.3  Amp. 
\  Curve  C  No  load  R.p.m.l300Arm.Current  6  Amps.         )  Field  Current- 
D  Rated  load  R.p.m.l050Arm.  Current  25  Ampe.r  =  .23  Amp, 


FIG.    9.  • —  FLUX   DISTRIBUTION   OF    5'H.P.    BULLOCK  MOTOR. 


An  examination  of  the  flux-distribution  diagram  (Fig.  9)  of  this 
motor  shows  firstly  how  much  the  field  flux  is  reduced  in  value  to 
obtain  the  highest  speed,  and,  secondly,  how  the  armature  distortional 
effects  have  forced  the  field  magnetism  to  the  left,  the  crossing-point 
or  zero  flux  value  being  no  longer  under  the  brush,  which  naturally 
causes  the  sparking  noted  above. 

The  efficiency  curves  (Fig.  10)  of  this  5-horsepower  motor  bring 
out  the  fact  that,  at  corresponding  loads,  the  efficiency  of  the  machine 
is  less  the  higher  the  speed.  This  is  to  be  expected  because  frictional 


52        ELECTRIC   MOTORS,    THEIR   ACTION  AND   CONTROL. 

losses  increase  more  rapidly  than  the  iron  losses  fall  off,  the  same 
being  true  generally  of  all  adjustable-speed  motors  unless  provided 
with  ball  or  roller  bearings. 


2  3  .4 

Horse -Power  Output 

FIG.   10.  —  EFFICIENCY  CURVES  OF   5-H.P.   BULLOCK  MOTOR  AT 
SPEEDS   OF   350,    700  AND    1050  R.P.M. 


Machines  of  Class  (b)  were  formerly  manufactured  by  the  Magneto 
Electric  Company,  and  called  Storey  Motors,  after  the  designer. 
These  motors  are  of  interest  because  they  show  how  the  concentra- 
tion and  holding  of  the  flux  at  the  pole  tips  can  be  obtained  by  simply 
hollowing  the  field  cores,  as  represented  in  Fig.  n. 


FIG.    II.  —  FIELD   FRAME   CONSTRUCTION   OF   STOREY  MOTOR. 


The  frame  of  the  motor  is  of  soft  steel  and  the  flux  density  high, 
but  not  reaching  the  bend  of  the  magnetization  curve,  as  Fig.  12 
shows;  and  the  cores  are  relatively  short. 


SPEED   CONTROL   OF  SHUNT   MOTORS. 


53 


The  data  of  a  3-h.p.,  3  :  i  adjustable-speed  Storey  motor  exam- 
ined by  the  authors  is  as  follows: 

Rated  pressure,  115  volts. 

Armature  current  at  rated  load,  25  to  28  amperes,  increasing  with 
the  speed. 

Field  current,  .5  min.,  1.7  max. 


120 
110 
1  nn 

/ 

/, 

f 

90 

80 

-2 
o70 

I60 

0> 

I50 

40 
30 
20 
10 

( 

// 

y 

4 

/ 

z 

/ 

/ 

/ 

/ 

/ 

430  I 

.p.m. 

/ 

V 

/ 

2 

// 

/ 

/ 

/ 

7_ 

)        .2       .4        .6       .8      LO      1.2      1.4      1.6     1.8      2.0     2.5 
Field  Amperes 

FIG.    12.  —  MAGNETIZATION   CURVE  OF   3'H.P.  STOREY  3  !    I    ADJUSTABLE-SPEED 

SHUNT  MOTOR. 

No-load  armature  current,  i.i  to  4  amperes,  increasing  with  the 
speed. 

Armature  resistance,  .31  ohm. 

Field  resistance,  67.5  ohms. 

Speed,  430  to  1290  r.p.m. 

Weight,  800  pounds. 

The  flux-distribution  curves  of  this  motor  (Fig.  13)  show  a  very 
uniform  flux  under  the  pole  pieces  at  minimum  speeds,  also  that  the 
flux  reversal  line  remains  fixed  independently  of  the  load,  thus  main- 
taining a  flux  for  commutation,  which,  however,  is  notably  decreased 
in  width  as  the  main  field  is  weakened,  causing  the  ultimate  develop- 


54        ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 


ment  of  sparking  as  well  as  poorer  speed  regulation.  This  latter 
fact  is  also  brought  out  by  a  study  of  the  speed-load  curves  of  this 
motor  in  Fig.  14. 


1° 


Minimum  Speed 
ad 
o  Load 


R.P.M.  =  430 
C8h=  1.7  Amp. 
C,  =  25  Amp. 


•1-2.0  • 


Maximum  Speed 
Rated  Load 


No  Load 


R.P.M.=1290     V\          / 
C.h  =  .5Amp.       \\       / 
C.=  28Amp.        \L/J 


C»'=  4  Amp. 


HG.  13. — FLUX-DISTRIBUTION  CURVES  OF  3'H.P.  STOREY  SHUNT  MOTOR. 
1600  f=^ 
1400  — 
1200 


1000 

;  800 

600 

400 
200 


Field  Curre 


t.5  Amp. 


1234 
Horse  Power  Output 

FIG.    14.  —  SPEED-LOAD  CURVES  OF  3-H.P.  STOREY  SHUNT  MOTOR. 

For  example,  the  drop  in  speed  from  no  load  to  rated  load  with  the 
weakest  field  is  29  per  cent,  whereas  the  decrease  in  speed  over  the 
corresponding  load  range  at  the  strongest  field  is  only  16  per  cent; 
thus  the  falling  off  in  speed  with  weakest  field  excitation  is  13  per 
cent  greater.  Only  a  small  part  of  this  is.  due  to  the  greater  IaRa 
drop  (28  X  .31  =  8.7  instead  of  25  X  .31  =  7.8);  hence  the  extra 
falling  off  in  speed  occurring  at  weakest  field  must  be  primarily  due 
to  poorer  contact  and  sparking  at  the  brushes. 


SPEED   CONTROL   OF  SHUNT  MOTORS. 


55 


The  efficiency  curves  of  this  Storey  motor  (Fig.  15)  show  that  the 
second  speed  (860  r.p.m.)  is  probably  the  best  for  average  service; 
while  the  highest-speed  curve  indicates  large  stray-power  losses. 


Horse  Power  Output 
FIG.    15.  —  EFFICIENCY  CURVES  OF  3'H.P.  STOREY  SHUNT  MOTOR. 

It  should  be  noted  in  the  case  of  these  preceding  types  of  adjust- 
able-speed shunt  motors  (a  and  b)  that  the  speed  in  all  cases  falls 
off  considerably  as  the  load  comes  on,  the  greatest  percentage  of 
reduction  occurring  with  the  weakest  field,  and,  in  all  cases,  the  drop 
in  speed  is  either  equal  to  or  greater  than  that  caused  by  IaRa  drop. 
Moreover,  as  the  brushes  of  these  machines  are  set  back  from  the 
geometrical  neutral  zone,  they  cannot  run  equally  well  in  both  direc- 
tions of  rotation,  without  brush  shifting. 

The  adjustable-speed  motors  (type  S)  of  the  Northern  Electric 
Manufacturing  Company  represent  a  construction  similar  in  prin- 
ciple to  that  of  the  above-described  Storey  motors  and  belong 
therefore  to  the  same  class  (b).  Their  pole  pieces  are  split  in  the 
direction  of  the  flux,  forming  a  field  frame  of  clover-leaf  form. 
This  frame,  built  up  of  laminations,  is  similar  to  the  construction 
shown  in  Fig.  17,  but  without  the  commutation  lugs. 

Class  (c) .  —  The  earlier  form  of  Thompson-Ryan  motor  diagram- 
matically  illustrated  in  Fig.  16  was  originally  brought  out  for 


56        ELECTRIC  MOTORS,   THEIR  ACTION  AND   CONTROL. 

constant  speed,  but  the  demand  for  an  adjustable-speed  motor  of 
wide  range  led  to  its  adoption  for  this  latter  service  as  well.  A 
machine  of  this  type,  however,  is  very  expensive  to  build  or  to  repair, 


FIG.    1 6.  —  ORIGINAL   FORM   OF   THOMPSON-RYAN  MOTOR. 

and  therefore  for  the  small  sizes  commonly  employed  to  drive  ma- 
chine tools  the  modified  design  shown  diagrammatically  in  Fig.  17 
was  developed  in  the  spring  of  1904.  This  modified  type  retains  the 
compensating  winding  and  commutation  lugs  of  the  earlier  patented 
design,  but  discards  the  inner  polar  ring  with  its  inherent  cost,  con- 
necting the  commutation  lugs  directly  to  the  field  yoke  and  placing 
the  compensating  winding  in  slots  formed  in  the  main  polar  faces. 

The  function  of  the  compensation  coils  C,  C  (Fig.  16),  in  series 
with  the  armature  winding,  is  primarily  to  prevent  the  distortion  of 
the  field  flux  and  thus  eliminate  brush  shifting  under  load ;  this,  how- 
ever, was  not  found  effective  to  prevent  sparking,*  therefore  the  com- 

*  Transactions  A.  I.  E.  E.,  March  20,  1895,  Vol.  XII. 


SPEED   CONTROL   OF  SHUNT  MOTORS.  57 

mutation  lug  was  introduced  to  provide  the  necessary  flux  for  reversal 
directly  at  the  armature  coils  undergoing  commutation;  thus  general 
compensation  and  local  commutation  phenomena  are  combined. 


FIG.    17.  —  MODIFIED  FIELD  FRAME  OF  THOMPSON-RYAN 
ADJUSTABLE-SPEED  MOTOR. 

The  data  of  a  3-h.p.  Thompson-Ryan  motor  of  the  modified  type  tested 
by  the  authors  are  as  follows: 

Line  voltage,  250  volts. 

Armature  current  at  rated  load,  11.4  to  12.2  amperes,  rising  with  the  speed. 

Field  current,  .28  to  1.15  amperes,  increasing  as  speed  falls. 

No-load  armature  current,  i.o  to  2.2  amperes,  rising  with  the  speed. 

Armature  resistance,  2.1  ohms.  Compensating  and  commutating  coils'  resist- 
ance, 1.17  ohms. 

Field  resistance,  200  ohms. 

Speed,  350  to  1400  r.p.m.,  depending  upon  field  strength.  Weight  complete, 
650  pounds. 

The  flux-distribution  curves  in  Fig.  18  show  that  the  armature 
reaction  is  reversed  as  load  comes  on,  the  leading  corner  being  weak- 
ened and  the  trailing  one  strengthened,  which  is  just  the  converse  of 
the  action  occurring  in  other  motors.  As  a  result  of  this  action,  the 
leading  corner  is  weakened  more  than  the  trailing  corner  is  strength- 


58        ELECTRIC  MOTORS,    THEIR   ACTION   AND    CONTROL. 

ened,  and  the  net  effect  is  a  diminution  of  the  field  strength  under 

load  increase,  with  corresponding  improvement  in  speed  regulation. 

The  reversal  of  armature  reaction  can  readily  be  carried  so  far  as 

to  produce  hunting  and  racing  with  large  increase  of  load,  especially 


Minimum  Speed 

Field  Current =1.15 


Maximum  Speed 

Field  Current  =  .28 


FlG.    1 8.  —  FLUX-DISTRIBUTION  CURVES    OF   THOMPSON-RYAN   3'H.P.   MOTOR. 

at  the  higher  speeds.  This  scheme  for  obtaining  very  constant  speed 
regulation  is  not,  however,  economically  developed,  since  the  weight 
of  copper  used  in  the  compensating  winding  is  approximately  twice 
that  used  in  the  armature  winding,  which  naturally  means  greater 
IaRc  drop,  PaRc  losses  and  heating. 

Interesting  features  of  this  design  are  the  extremely  small  air  gap 
and  the  very  high  average  potential  difference  of  over  20  volts  exist- 
ing between  adjacent  commutator  bars.  In  fact  this  voltage  is 
undoubtedly  much  more  than  in  some  cases,  because,  when  the 
motor  is  operating  at  the  higher  speeds,  the  points  of  very  high 
flux  density  can  be  approximately  located  by  the  lines  of  scin- 
tillation on  the  commutator,  due  to  incipient  sparking  between 
neighboring  bars. 

The  speed-load  curves  of  this  motor  (Fig.  19)  represent  both 
clock-  and  counter-clock-wise  rotation  and  show  just  the  reverse  of 
the  characteristic  regulation  of  the  ordinary  simple  field-frame  shunt 
motors  (types  a  and  b) ,  in  that  the  speed  decrease  under  load  is  more 
pronounced  at  the  low  than  at  the  higher  speeds.  This  improve- 
ment in  regulation  is  due  to  the  field  distortion  and  reduction  caused 
by  the  action  of  the  balancing  windings.  For  example,  at  minimum 
speed  the  drop  in  speed  from  no  load  to  rated  load  is  14.5  per  cent, 
which  is  substantially  that  which  occurs  through  IaRa  drop.  At  the 
highest  rate  of  rotation  (field  current  =  .28  amps.)  the  decrease  in 
speed  from  no  load  to  rated  load  is  only  7.1  per  cent,  whereas  it. 


SPEED   CONTROL   OF   SHUNT   MOTORS. 


59 


1600 
1400 

1200 
1000 
800 
600 
400 

200 
0 


I«A=. 28  Am 


I9h=  1.15  Amps. 


1  2  %        3  4 

H.  P.  Load 

FlG.    19.  —  SPEED -LOAD  CURVES  OF  3'H.P.  THOMPSON-RYAN  MOTOR. 
8G 


TO 


60 


D. 


JC. 


v. 


B. 


•!•* 


A  l.}5  Amps. 

B 

0     .3 


Horse-Power  Load 
FlG.    20.  —  EFFICIENCY  CURVE  OF  3-H.P.   THOMPSON-RYAN  MOTOR. 

would  be  15  per  cent  due  to  IaRa  drop;  so  that  speed  regulation  is 
improved  8  per  cent  by  the  effect  of  the  balancing  winding. 

The  flux-distribution  curves  shown  in  Fig.  18  indicate  not  only 
very  markedly  the  reversal  of  armature  reaction,  but  also  the  net 


60        ELECTRIC  MOTORS,    THEIR   ACTION   AND   CONTROL. 

decrease  of  main-field  strength  occurring  at  the  highest  speed.  The 
falling  off  of  flux  to  zero  and  even  reversal,  under  the  middle  of  the 
main  poles,  is  caused  by  the  fact  that  the  laminated  construction 
employed  has  the  main  poles  split  from  the  pole  face  back  through 
the  yoke.  This  construction  is  necessary  so  that  the  removal  of  the 
various  field  coils  for  repair  is  feasible. 

The  efficiency  curves  of  this  motor  in  Fig.  20  indicate  nothing 
unexpected,  because  it  is  obvious  from  the  construction  that  copper 
losses  are  great,  and  the  flux-distribution  curves  show  that  the  core 
losses  are  also  large,  on  account  of  the  crowding  and  numerous 
reversals  of  flux. 

This  type  of  motor  is  extremely  sensitive  to  change  of  brush  posi- 
tion, a  barely  perceptible  movement  forward  or  backward  producing 
quite  different  speed  characteristics,  so  that  the  machine  runs  faster 
in  the  clockwise  direction  of  rotation  or  more  slowly  in  the  opposite 
direction,  acting  in  the  one  instance  like  a  differential  motor  and  in 
the  other  like  a  heavily  over-compounded  motor.  The  ordinary 
wear  of  the  brushes  during  use  or  the  formation  of  even  invisible 
sparks  under  the  brushes,  which  is  not  unlikely  to  occur,  alters  the 
speed  regulation  considerably  and  frequently  leads  to  more  pro- 
nounced and  objectionable  sparking. 

Class  (d). — Inter  pole*  or  commutation-pole  motors  (Fig.  21)  con- 
stitute what  is  herein  designated  as  Class  (d)  of  adjustable-speed 
motors.  In  such  machines  auxiliary  poles  are  introduced  between 
the  main-field  poles.  These  interpoles  are  excited  by  coils  connected 
in  series  with  the  armature,  so  that  full  or  proportional  part  of  the 
armature  current  flows  through  them.  This  type  differs  from  Class 
(c)  in  that  the  compensation  winding  is  discarded  and  armature 
reaction  is  therefore  not  eliminated  or  reversed,  a  local  commutation 
flux  alone  being  depended  upon  for  sparkless  operation  throughout 
the  range  of  speed.  In  fact,  with  this  type  armature  reaction  is 
actually  exaggerated  because  the  flux  from  the  interpole  strengthens 
the  leading-pole  corner  and  weakens  the  trailing-pole  corner  just  as 
the  armature  m.m.f.  does.  This  exaggeration  of  field  distortion 
does  no  harm,  but,  on  the  contrary,  it  improves  the  speed  regulation 
of  the  machine.  The  interpolar  flux  for  reversal  is  independent  of 
the  main  field ;  being,  however,  directly  dependent  upon  the  armature 

*  U.  S.  Patent  No.  775,310,  November  22,  1904. 


SPEED    CONTROL   OF   SHUNT   MOTORS. 


61 


current,  it  increases  therewith  and  thus  maintains  the  necessary 
commutating  field. 

The  connections  of  this  type  of  motor  are  diagrammatically  indi- 
cated in  Fig.  22,  N,  5,  N,  S  being  the  main  poles,  and  n,  s,  n,  s 
the  interpoles.  Each  interpole  is  of  the  same  polarity  as  that  of 


FIG.    21.  —  INTERPOLAR  MOTOR  —  FRONT  BEARING  REMOVED. 


FIG.    22.  —  CONNECTIONS  OF  INTERPOLAR  MOTOR. 

the  main  pole  immediately  back  of  it,  depending  upon  the  direction 
of  rotation;  hence  the  illustration  shows  polarities  for  clockwise 
rotation.  As  represented,  the  interpoles  are  small  with  respect  to 
the  main-field  poles,  the  arc  of  armature  periphery  subtended  by 


62         ELECTRIC   MOTORS,    THEIR   ACTION   AND    CONTROL. 

the  former  being  about  one-sixth  that  embraced  by  the  latter. 
As  already  stated,  the  interpoles  are  provided  with  magnetizing 
coils  connected  in  series  with  the  armature  and  are  placed  midway 
between  the  main  poles,  directly  over  the  armature  coils  under- 
going commutation.  The  commutator  brushes  are  consequently 
so  set  that  they  short-circuit  coils  in  the  geometrical  neutral  posi- 
tion. With  this  setting  of  brushes  the  motor  will  operate  with 
substantially  the  same  speed  characteristics  in  either  direction  of 
rotation. 

Data  of^  a  5-h.p.,  6  to  i  variable-speed  interpolar  motor  manu- 
factured by  the  Electro-Dynamic  Company,  and  tested  by  the 
authors,  are  as  follows: 

Rated  voltage,  240. 

Armature  current  at  5-h.p.  output,  22.2  to  24  amperes,  increas- 
ing with  speed. 

Armature  current  running  free,  .7  to  1.7  amperes,  increasing 
with  speed.  Field  current,  adjusted  between  1.27  and  .16  amperes 
to  obtain  speeds  from  210  to  1260  r.p.m.  at  5-h.p.  output. 

Resistance  of  armature  winding,  .9  ohm  at  75  degrees  C. 

Resistance  of  interpole  winding,  .2  ohm  at  75  degrees  C. 

Resistance  of  shunt-field  winding,  176  ohms  at  75  degrees  C. 

Speed  205  to  1260  r.p.m.  at  5-h.p.  output,  increasing  with  weaker 
field. 

Weight,  1200  pounds. 

The  magnetization  curve  of  this  motor  (Fig.  23)  shows  that,  for 
the  minimum  speed,  the  flux  density  is  carried  well  up  above  the 
bend;  this  is  also  quite  apparent  from  the  fact  that  a  speed  ratio 
of  i  :  6  is  obtained  with  field  currents  at  8  :  i. 

The  speed-load  curves  of  this  motor  (Fig.  24)  indicate  excellent 
speed  regulation,  with  an  actual  increase  of  speed  under  load  at 
the  weakest  field  value.  The  regulating  influence  of  armature  and 
interpole  reaction  upon  the  main  magnetic  field  and  speed  is 
brought  out  by  the  following  examples: 

The  no-load  speed  with  field  current  of  1.27  amperes  is  222  r.p.m. 
The  speed  diminution  caused  by  IR  drop  is  22  r.p.m.;  nevertheless, 
at  rated  load  with  field  current  of  1.27  amperes,  the  speed  is  205 
r.p.m.;  hence  the  effect  of  armature  and  interpole  reaction  is  to 
hold  up  the  speed  5  r.p.m.  compared  with  what  it  would  otherwise 
be.  At  a  speed  of  740  r.p.m.  the  armature  reaction  exactly  com- 


SPEED    CONTROL   OF   SHUNT   MOTORS, 
i 


220 
200 
180 
160 
140 

I'20 
100 

80 
60 
40 
20 
n 

x 

< 

/ 

r 

// 

?K.r- 

m.  210 

// 

I 

/ 

V 

/ 

/A 

f 

// 

// 

'/ 

Field  Current 
FIG.    23.  —  MAGNETIZATION  CURVE  OF  5~H.P.  INTERPOLAR  SHUNT  MOTOR. 


HOO 
1200 
1000 

j£,800 
P3 
600 

400 
200 


I«ft=.16  Amp. 


1.27  Amp, 


12345678 
Horse-Power  Output 

FIG.  24.  —  SPEED-LOAD  CURVES  OF  5~H.P.   INTERPOLAR  SHUNT  MOTOR, 
SPEED  RANGE  6  I  I. 


64        ELECTRIC   MOTORS,    THEIR   ACTION   AND    CONTROL. 


pensates  for  IR  drop  and  the  motor  speed  remains  constant  up  to 
the  rated  output  of  5  horsepower.  With  field  current  of  .16  am- 
pere the  motor  speed  rises  from  1209  to  1260  r.p.m.  when  output 
increases  from  zero  to  5  horsepower.  Apparently  it  should  fall 
from  1209  to  1084  r.p.m.,  which  is  the  ratio  between  the  c.e.m.f's 
(i.e.,  238.1  :  213.6),  but  the  reaction  on  the  main  field  by  the  inter- 
poles  and  armature  weakens  the  same  sufficiently  to  raise  the  speed 
176  r.p.m. 


Arm.  Current  .72  Amp. 
„  "     22.2      " 

I«ft     1.27    « 


FIG.   25A.  —  FLUX  DISTRIBUTION  OF  5~H.P.    ADJUSTABLE-SPEED  INTERPOLAR 
SHUNT  MOTOR,  FIELD  CURRENT   1.27  AMPS.;  SPEED  AT  RATED  LOAD  2IO  R.P.M. 

It  is  thus  evident  that  with  this  type  of  machine  the  speed  regu- 
lation is  the  reverse  of  that  obtained  with  adjustable-speed  motors 
having  the  ordinary  forms  of  field  magnet  (Classes  a  and  b). 

The  interpolar  type  of  motor  can  be  readily  reversed  in  direction 
of  rotation  even  while  under  load,  on  account  of  the  great  self- 
induction  of  the  interpole  and  armature  circuit  and  the  production 
of  the  proper  value  of  commutation  flux. 

The  flux-distribution  curves  in  Fig.  25 A  indicate  that  at  strong 
field  excitation  the  interpoles  do  not  produce  a  very  great  effect. 
The  same  fact  was  also  shown  in  the  speed-load  examples  on  page 
62.  With  small  field  flux,  however  (Fig.  253),  the  interpole  m.m.f. 
and  armature  reaction  produce  a  marked  weakening  and  distor- 
tion of  the  main  field,  which  phenomena  are  also  apparent  from 
the  speed-load  curves  (Fig.  24). 

If  the  brushes  be  displaced  from  the  geometrical  neutral  position, 
the  motor  speed  is  considerably  changed.  For  example,  with  the 
brushes  shifted  backward  (opposite  to  rotation  direction),  the  speed 


SPEED   CONTROL   OF  SHUNT   MOTORS. 


65 


will  rise  under  load,  because  then  the  interpolar  flux  develops  in 
the  armature  an  e.m.f.  which  decreases  that  produced  by  the  main- 
field  poles.  If  the  brushes  are  advanced  in  the  direction  of  rota- 


Arm.  Current  1.7  Amp. 

«       24.0      « 
L.A        =         .16    « 


FIG.    256.  —  FLUX  DISTRIBUTION  OF  5-H.P.  ADJUSTABLE-SPEED  INTERPOLAR 
MOTOR,  FIELD  CURRENT  .16  AMP.;    SPEED  AT  RATED  LOAD   1260  R.P.M. 

tion  the  speed  will  fall  under  load,  as  in  the  case  of  a  cumulative 
compound  motor,  because  the  interpolar  flux  generates  an  e.m.f. 
in  the  same  direction  as  that  due  to  the  main  poles.  In  conse- 
quence of  this,  the  proper  position  for  the  brushes  is  obtained  when 


FIG.  26.  —  EFFICIENCY  CURVES  OF  A  6-1  ADJUSTABLE-SPEED  "iNTERPOLE"  MOTOR. 


the  r.p.m.  are  the  same  in  both  directions  for  given  load  and  field 
strength,  and  motors  of  this  type  should  not  be  removed  from  the 
factory  test  table  until  such  adjustment  is  secured. 


66        ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

The  efficiency  curves  of  this  motor  (Fig.  26)  indicate  high  values 
for  light  loads,  which  is  due  to  the  use  of  ball  bearings.  The  rapid 
falling  off  in  efficiency  after  rated  load  is  reached  is  to  be  expected, 
on  account  of  the  additional  PR  losses  caused  by  the  interpole 
windings.  The  best  running  speed  of  this  motor  is  apparently  at  a 
field  strength  of  about  .3  ampere,  because  at  this  value  the  general 
efficiency  of  the  motor  is  considerably  greater  than  at  lower  or  higher 
speeds. 

Dunn  Method.  —  Another  type  of  motor,  the  speed  of  which  is  ad- 
justable by  varying  field  flux,  was  invented  by  Mr.  Gano  S.  Dunn.* 
The  armature  is  supplied  with  constant  current  and  the  field 
winding  separately  excited  from  a  constant-potential  circuit  through 
a  rheostat.  The  armature  current  being  constant,  the  torque  varies 
directly  with  field  flux.  By  means  of  the  field  rheostat  this  flux 
may  be  regulated  from  a  very  low  value  up  to  full  strength  with 
corresponding  increase  of  torque.  This  large  range  of  control  is 
obtained  by  regulating  the  field  current,  which  is  small,  the  heavy 
armature  current  being  kept  constant  by  an  automatically  regu- 
lated generator,  as  in  constant-current  arc  lighting.  The  advan- 
tage is  similar  to  that  secured  by  the  " field-weakening"  method 
already  described,  but  gives  any  torque  or  speed  from  zero  to  full 
value,  while  the  latter  is  practically  limited  (unless  special  designs 
are  employed)  to  a  certain  ratio  of  speeds,  usually  2  or  3  to  i. 
This  method  possesses  an  additional  advantage  over  field-weak- 
ening control  in  having  maximum  field  strength  with  maximum 
speed  and  torque.  In  these  respects  it  would  be  adapted  to 
adjustable-speed  work  in  machine  shops.  On  the  other  hand, 
the  necessity  for  constant-current  as  well  as  constant-potential 
supply,  and  the  high  voltage  required  to  give  any  considerable 
power,  are  serious  objections  to  this  system.  It  is  hardly  prac- 
ticable to  operate  motors  below  20  horsepower  with  more  than 
100  amperes,  at  which  current  it  would  require  about  1000  volts 
to  supply  100  horsepower  on  one  circuit  —  a  dangerous  voltage  in 
a  shop.  To  multiply  circuits  is  objectionable  because  each  would 
demand  its  separate  constant-current  generator.  Furthermore,  the 
latter  has  not  been  developed  commercially  above  10  amperes. 
For  these  reasons,  the  field-weakening  and  multiple-voltage  methods 
are  preferred  for  machine  shop  or  similar  service. 

*  U.  S.  Patents  No.  549,061,  October  29,  1895,  and  No.  591,345,  October  5,  1897. 


CHAPTER   VII. 

SPEED    CONTROL   OF   MOTORS   BY   VARIATION   OF  FIELD 
RELUCTANCE. 

THE  preceding  chapter  dealt  with  the  problem  of  shunt-motor 
speed  adjustment  by  variation  of  the  field-exciting  current;  this 
chapter  is  descriptive  of  those  motors  whose  speed  regulation  de- 
pends upon  the  variation  of  the  reluctance  of  the  magnetic  circuit. 
This  method  of  control  is  based  upon  the  fundamental  fact  that 
magnetomotive  force 
reluctance 


flux  = 


Thus,  if  the  m.m.f.  be  maintained 


FIGS.    27    AND    28.  —  METHODS    OF    VARYING    RELUCTANCE    OF    THE    MAGNETIC 

CIRCUIT. 


constant  and  the  reluctance  be  varied,  the  field  flux  is  changed  in  the 


inverse  manner,  and  from  the  relation  r.p.m.  = 


e  io86o  b 
$>n  2  p 


it  is  evident 


that  the  speed  varies  inversely  as  the  field  flux  (<I>)  or  in  the  same 
ratio  as  the  change  in  the  reluctance.  Among  the  earlier  methods 
tried  were  those  of  T.  A.  Edison  and  the  Diehl  Company. 

The  Edison  Variable  Reluctance  Methods  of  Control.  -  -  The  re- 
luctance of  the  magnetic  circuit  in  the  machine  was  varied,  as 

67 


68        ELECTRIC   MOTORS,    THEIR   ACTION   AND   CONTROL. 

shown  in  Fig.  27,  by  decreasing  the  amount  of  metal  in  the  yoke 
of  the  field  magnet,  the  wedge-shaped  piece,  A,  being  raised,  thus 
decreasing  the  total  flux.  The  range  of  speed  adjustment  is  limited, 
however,  as  excessive  sparking  develops  when  the  field  is  weakened 
because  there  is  no  feature  of  design  to  prevent  flux  distortion.  This 
method  was  primarily  intended  for  voltage  regulation  in  connection 
with  generators,  and  is  of  historical  rather  than  commercial  impor- 
tance. 

The  Diehl  Method  of  Control.  —  In  this  type  of  machine  flux  re- 
duction was  obtained  by  a  lengthening  of  the  air-gap.  The  field 
magnet  was  hinged  so  that  the  pole  pieces  could  be  moved  away 
from  the  armature  as  indicated  in  Fig.  28.  This  construction  was 
not  very  successful  and  was,  like  the  preceding,  originally  intended 
as  a  means  for  regulating  the  voltage  of  generators. 

Two  modern  methods  of  speed  control  by  variation  of  reluctance 
are  those  of  the  Stow  Electric  Company  and  the  Lincoln  Manufactur- 
ing Company. 


FIG.    29.  —  STOW  ADJUSTABLE-SPEED   MOTOR. 

The  Stow  Adjustable-speed  Motor.  —  The  speed  increase  of  this 
machine  also  depends  upon  the  removal  of  iron  from  its  magnetic 
circuit,  the  pole  cores  being  made  hollow  and  provided  with  iron 
or  steel  plungers,  the  position  of  which  is  made  adjustable  through 
worm  gears  and  pinions  operated  by  the  large  hand-wheel,  at  the  top, 


SPEED   CONTROL   OF   MOTORS. 


69 


f 


~L 


P  unger 


R.p.m.' 


u  .1  ^  .3,  .4  A  A  J 

Field  Amperes 

FIG.    30.  —  MAGNETIZATION  CURVES  OF  4-H.P.  STOW  MOTOR. 


2400 
£200 
2000 

i1800 

^1600 
1100 
1200 
1000 
800 

leoo 

400 
200 


nger  C 


Horse- Power  Output 
FIG.  31.  —  SPEED-LOAD  CURVES  OF  4-H.P.  STOW  MOTOR. 


70        ELECTRIC   MOTORS,    THEIR   ACTION   AND    CONTROL. 

as  represented  in  Fig.  29.*  When  the  plungers  are  withdrawn, 
the  total  flux  decreases  because  of  the  lengthening  and  reduc- 
tion of  area  of  the  effective  air-gap,  also  the  decrease  of  effective 
metal  in  the  field  cores.  The  flux  being  now  along  the  polar 
edges  (in  varying  degree  according  to  the  position  of  the  plungers), 
the  field  for  commutation  is  maintained  relatively  strong,  and  spark- 
ing thereby  prevented.  The  concentration  of  flux  at  the  polar  edges 
is  well  shown  by  the  flux-distribution  curves  (Fig.  33)  of  this  ma- 
chine. On  the  other  hand  these  curves  also  show  that  armature 
reaction  forces  the  commutation  fringe  back,  at  the  higher  speeds 
and  loads;  consequently,  to  maintain  sparkless  commutation  at  the 
3  to  i  range  of  speed,  the  brushes  must  be  given  a  lag.  This  requires 
the  brushes  to  be  shifted  if  the  direction  of  rotation  is  reversed,  or  if 
the  speed  range  is  greater  than  3  to  i.  It  is  a  fact, however, that  since 
metal  is  removed  the  effect  of  armature  reaction  for  a  given  current 
is  reduced  because  the  reluctance  of  the  path  of  the  armature  flux 
is  increased.  The  data  of  a  3  to  i  adjustable-speed  4-h.p.  motor 
of  this  Stow  type  are  as  follows: 

Rated  pressure,  220  volts. 

Armature  current  at  rated  output  of  4  h.p.,  16.75  to  I7-°  amperes, 
increasing  with  speed. 

Field  current  constant  at  .64  ampere. 

No-load  armature  current,  1.3  to  2.2  amperes,  increasing  with 
speed. 

Field  resistance,  "hot,"  344  ohms. 

Speed,  725  r.p.m.  min.  to  2175  r.p.m.  max. 

Weight,  800  pounds. 

The  operation  of  this  machine  under  various  loads  and  speeds  is 
shown  in  the  curves,  Figs.  30,  31,  32,  and  33,  which  represent,  respec- 
tively, magnetization,  speed-load  relations,  efficiency,  and  flux  dis- 
tribution. A  study  of  the  speed-load  curves  indicates  that  the  speed 
regulation  at  the  higher  rates  of  rotation  is  not  as  good  as  that  with 
the  stronger  fields.  This  is  due  to  the  fact  that  invisible  sparking  at 
the  brushes  and  poorer  brush  contact  increase  the  IaRa  drop,  just  as 
in  the  types  (a)  and  (b)  adjustable-speed  motors  which  are  controlled 
by  variation  of  field  current.  The  most  efficient  operating  speed 
of  this  motor  is  at  the  second  plunger  adjustment,  which  gives 
1090  r.p.m. 

*  U.  S.  Patents  Nos.  666,315  and  672,419,  January  and  April,  1901. 


SPEED   CONTROL   OF  MOTORS. 


71 


' 


B  IF' 


p.m. 


K_l 

o 


23 
Horse-Po^ver  Output 


FIG.    32.  —  EFFICIENCY  CURVES  OF  4~H.P.  STOW  MOTOR. 

A  710  r.p.m.  at  rated  load. 
B  1090  r.p.m.  at  rated  load. 
C  1490  r.p.m.  at  rated  load. 
D  2130  r.p.m.  at  rated  load. 


10- 


6- 


Plungers  in 

'/  Field -Current  .64  Amps. 

Ann.-Cunent  1.3      " 

"       16.7      " 
R.p.m  726. 


Plungers  out 

Field-Current  .64.  Amps. 
Ann.-Current  2J2      <• 


FIG.    33. FLUX  DISTRIBUTION  CURVES  OF  4-H.P.   STOW  MOTOR. 


72        ELECTRIC  MOTORS,   THEIR  ACTION   AND    CONTROL. 

The  Lincoln  Adjustable-speed  Motor.  — The  variation  in  reluc- 
tance of  the  magnetic  circuit  of  this  type*  is  obtained  both  by 
lengthening  the  air-gap  and  by  decreasing  its  effective  area.  The 
armature  is  formed  as  a  truncated  cone  with  corresponding  polar 


FIG.  34.  —  LINCOLN  ADJUSTABLE -SPEED  MOTOR. 


012345        67        89       10      11      12 


FIG.    35.  —  SPEED-LOAD  CURVES  OF  IO-H.P.  LINCOLN  MOTOR. 

surfaces  as  represented  in  Fig.  34.  The  armature  is  movable  in 
the  direction  of  its  axis,  so  that  movement  one  way  increases  the 
length  of  the  air-gap,  thus  decreasing  the  flux  and  raising  the  motor 
speed.  The  fact  that  the  effective  length  of  the  armature  inductors 

*   U.  S.  Patent  No.  829,974,  September,  1906. 


SPEED   CONTROL   OF   MOTORS. 


73 


within  the  magnetic  field  is  decreased  by  this  shifting  of  the  arma- 
ture also  increases  the  motor  speed. 

The  characteristic  working  curves  of  such  a  machine  of  10  horse- 
power and  5  to  i  speed  range  are  given  in  Figs.  35  and  36,  being 
those  of  speed-load  and  efficiency  at  various  loads,  respectively. 
The  principal  objections  to  this  construction  are  the  extra  space 
required  for  the  armature  and  the  large  force  required  to  move  it. 


0        1       2        3        4       5        6        7       8        9       10      U 

Horse  Power  Output 

FIG.    36.  —  EFFICIENCY  CURVES  OF  IO-H.P.  LINCOLN  MOTOR 

The  flux-distribution  curves  of  the  motors  are  similar  to  those  of 
ordinary  single-speed  machines,  the  flux  distortion  at  high  speeds 
being  limited  on  account  of  the  increase  of  the  air-gap  lengths. 
However,  to  ensure  sparkless  operation  at  the  high  speeds,  interpoles 
in  series  with  the  armature  are  employed  in  the  more  recent  designs. 
It  is  to  be  noted  that  in  this  machine,  as  with  types  (c)  and  (d)  motors 
having  field-rheostat  control,  the  speed  regulation  is  better  with  weak 
than  with  the  stronger  magnetic  fields,  the  variation  in  r.p.m.  being 
2.3  and  6  per  cent  respectively. 


CHAPTER    VIII. 

MULTIPLE- VOLT  AGE   CONTROL   OF   MOTOR    SPEED. 

THE  immediately  preceding  chapters  were  devoted  to  a  consid- 
eration of  the  "  field-weakening  "  methods  of  motor-speed  adjustment. 
This  chapter  and  the  one  that  follows  set  forth  the  remaining 
methods  of  adjusting  the  speed  of  direct-current  shunt  motors. 
These  differ  from  the  foregoing  in  that  they  depend  upon  altera- 
tion of  impressed  voltage,  and  are  therefore  called  adjustable  or 
multi-voltage  systems  in  contradistinction  to  the  constant-  or  single- 
voltage  systems  already  described.  In  other  words,  the  speed 
adjustment  is  due  to  changes  external  to  instead  of  internal  with 
respect  to  the  motor.  These  methods  also  differ  from  the  preced- 
ing in  the  fact  that  they  can  produce  a  constant  torque  over  the 
entire  speed  range  (with  the  single  exception  noted  below)  in 
place  of  a  constant  output  in  horsepower.  The  general  classifica- 
tion of  these  adjustable-voltage  methods  is  as  follows: 

1.  Armature  rheostat  — already  described. 

2.  Multiple-voltage  (multiple-wire")  systems. 

3.  Motor-generator  systems. 

4.  Boost  and  retard  systems. 

5.  Teaser  systems. 

6.  Double-armature  motors. 

7.  Variation  of  number  of  poles  in  motor. 

In  the  last  two  of  the  above  cases,  Nos.  6  and  7,  the  motor  as  a 
whole  is  supplied  with  constant  voltage.  Nevertheless  the  volt- 
age available  for  each  armature  is  varied  in  case  6,  and  the  group- 
ing of  the  armature  conductors  is  altered  in  case  7  so  as  to  change 
i  he  c.e.m.f.  developed;  hence  these  two  rather  peculiar  cases  may 
be  included  in  a  general  way  under  adjustable-voltage  control. 
They  also  resemble  the  other  five  cases  in  the  fact  that  field  cur- 
rent and  flux  are  usually  maintained  constant.  The  double-arma- 
ture motors  of  case  6  do  not  exert  constant  torque,  as  in  the  other 
six  cases,  but  produce  constant  output  in  horsepower  like  the 
field-weakening  types  already  explained.  It  is  also  to  be  noted 

74 


MULTIPLE-VOLTAGE   CONTROL   OF   MOTOR  SPEED. 


75 


that  cases  2,  6  and  7  in  the  above  list  apply  to  multi-speed  motors 
as  denned  by  the  A.  I.  E.  E.  Standardization  Rules  (Section  E, 
paragraph  48,  page  7)  rather  than  to  adjustable-speed  machines, 
which  latter  include  alfthose  heretofore  described. 

In  the  discussion  of  the  relation  between  speed  and  c.e.m.f.  it 
was  shown  that,  if  other  conditions  remain  constant,  the  speed 
varies  directly  as  the  c.e.m.f.  If  we  now  examine  the  equation 
V  =  c.e.m.f.  +  (IaRa  +  Db)  it  is  evident  if  IaRa  and  Db  are  small 
with  respect  to  the  c.e.m.f.  (as  must  be  the  case  with  an  efficient 
motor)  that  increase  in  V  will  produce,  at  constant  torque,  a  nearly 
proportional  increase  in  c.e.m.f.,  that  is,  a  variation  of  impressed 
e.m.f.  produces  a  corresponding  change  in  speed.  This  is  the 
principle  of  the  adjustable-voltage  or  multi-voltage  systems  of 
control. 

Three-wire  Multiple-voltage  Systems.  — The  simplest  multiple- 
voltage  system  is  the  ordinary  three- wire  circuit,  with  say  115  volts 
between  either  outer  and  the  neutral  conductor,  and  230  volts 
between  the  outer  conductors  as  represented  in  Fig.  37.  Thus  a 
230-volt  shunt  motor  connected  so  that  its  field  winding  is  supplied 
with  230  volts  and  its  armature  with  115  volts  develops  a  certain 


v. 


230  V 


V. 


FIG.    37.  —  SIMPLE  THREE-WIRE  MULTIPLE-VOLTAGE  SYSTEM. 

speed.  If  the  armature  terminals  are  then  connected  to  the  230- 
volt  supply,  the  speed  will  be  approximately  twice  as  great.  The 
two  principal  running  points  are  nearly  one-half  and  full  speed, 
while  those  intermediate  may  be  obtained  by  the  introduction  of 
armature  rheostat  or  " field-weakening"  control.  Let  us  consider 
the  lo-h.p.  motor,  the  data  of  which  were  given  on  page  20. 
This  machine  has  an  armature  resistance,  hot,  of  .28  ohm;  a  brush 
drop  of  1.4  volts  and  an  armature  current  of  37  amperes  at  rated 
load. 


76        ELECTRIC  MOTORS,    THEIR   ACTION   AND   CONTROL. 

Then  IaRa  +  Db  =  .28  X  37  +  1.4  =  n.8  volts.  With  V  =  115, 
the  c.e.m.f.  is  115  —  n.8  =  103.2.  With  V  =  230,  the  c.e.m.f. 
is  230  —  1 1. 8  =  218.2.  Hence  the  speed  ratio  at  these  voltages 
is  as  103.2  :  218.2,  or  nearly  a  i  to  2  speed  change  (i  :  2.11). 

The  objection  to  this  method  is  that  while  three  wires  are  neces- 
sary, only  two  running  speeds  are  obtained.  An  additional  speed 
can  be  secured  from  an  unsymmetrical  three-wire  system,  in  which 
one  of  the  sides  has  a  voltage  of  x  and  the  other  side  a  voltage  of 
2  x\  but  even  then  only  three  running  speeds  corresponding  to 
x  :  2  x  :  3  x  could  be  obtained  with  three  wires.  To  gain  a  much 
wider  speed  range  with  only  one-third  greater  number  of  wires, 
the  four-wire  systems  were  developed,  and  these  will  now  be 
explained. 

Ward  Leonard  Multiple-voltage  Control  of  Speed.  — The  first  of 
these  four-wire  methods,  historically,  is  that  of  H.  Ward  Leonard,* 
who  employed  three  generators  of  62,  125  and  250  volts,  respectively, 
and  grouped  them  in  series  in  the  order  named,  as  represented  in 
Fig.  38.  These  voltages  were  supplied  to  the  various  motors  by  a 
four-wire  system  of  distribution,  connected  to  the  three  generators 
in  the  manner  shown. 


FlG.    38.  —  WARD  LEONARD  MULTIPLE-VOLTAGE  SYSTEM. 

The  shunt-field  windings  of  all  the  motors  were  supplied  with  a 
constant  voltage,  either  the  total  amount  obtained  by  connection 
with  the  outside  wires  A  and  D,  or  a  smaller  value,  as,  for  example, 
that  existing  between  the  wires  C  and  D.  The  armature  terminals 
may  be  connected  as  desired  to  any  two  of  the  conductors  A,  B,  C 

*  U.  S.  Patent  No.  478,344,  July,  1892. 


MULTIPLE-VOLTAGE   CONTROL    OF   MOTOR  SPEED. 


77 


and  D',  thus  if  applied  to  A  and  B,  62  volts  would  be  obtained; 
across  B  and  C,  125  volts;  across  A  and  C,  167  volts;  across  C  and  £>, 
250  volts;  across  B  and  D,  375  volts;  and  across  A  and  Z>,  437  volts. 
Taking  the  speed  at  the  highest  voltage  as  the  full  or  rated  value, 
the  various  running  points  would  give  speeds  of  approximately  ^-, 
T>  f>  T>  f'  and  y>  a  sudden  jump  in  the  voltage  increment  occurring 
at  the  fifth  point.  The  -f-  speed  value,  or  that  corresponding  to  312 
volts,  could  not  be  obtained,  because  to  get  this  voltage  AB  would 
have  to  be  added  directly  to  voltage  CD,  which  would  short-circuit 
the  voltage  BC. 

Crocker-Wheeler  System.  —  The  next  four-wire  multiple-voltage 
system  developed  was  that  of  the  Crocker-Wheeler  Company, 
employing  voltages  of  40,  120  and  80  in  th3  order  given.  (Fig.  39.) 
By  connecting  the  field  terminals  across  the  24o-volt  lines  (AD) 
and  shifting  the  armature  terminals  from  AB  to  CD,  to  BC,  to 


v. 


V. 


240  V. 


160  V. 


120  V. 


V. 


240V. 


23456 
Running  Points 


FIG.    39.  —  CROCKER-WHEELER  MULTIPLE-VOLTAGE  SYSTEM. 

AC,   to    BD,    and   finally  to    AD,    six   voltages   and    speeds    are 
obtained  as  follows: 


AB  gives 40  volts 

CD    "      80    " 

BC    "  ..120    " 


AC  gives 160  volts 

BD     "      200     " 

AD     "  ..240    " 


These  voltages  correspond  approximately  to  jr,  J-,  -|,  -|,  -|  and  -§-  of 
the  rated  speed.  Thus,  with  this  system,  the  speeds  increase  in  a 
straight  line,  or  in  an  arithmetical  progression,  there  being  no  jumps, 
but  a  uniform  rise  throughout.  The  actual  speeds  are  from  106  to 
862  r.p.m.,  giving  a  range  of  a  little  more  than  i  to  8,  as  shown  in  the 
table  of  " Speeds  and  Efficiencies"  on  page  79. 


78        ELECTRIC   MOTORS,    THEIR  ACTION  AND   CONTROL. 

Bullock  Multiple-voltage  System.  —  A  third  multiple-voltage 
method  is  that  of  the  Bullock  Company,  Fig.  40,  employing  vol- 
tages which  increase  in  geometrical  progression  ;  that  is,  the  voltages 
are  in  the  following  ratio:  a  :  ar  :  ar2:  ar3:  ar*:  ar5.  As  these 
values  must  all  be  obtained  in  practice  from  a  single  system  consist- 
ing of  only  four  conductors,  it  is  necessary  that  ar 3  =  a  +  ar,  that 
ar*  =  ar  +  ar2  and  that  ar5  =  ar2  +  ar3  =  a  +  ar  +  ar2. 


250  V. 


12345 
Running  Points 

FIG.   40.  —  BULLOCK  MULTIPLE-VOLTAGE   SYSTEM. 

The  only  factor  r  which  satisfies  these  conditions  is  1.3247;  that 
is,  each  voltage  is  324  per  cent,  or  about  one-third,  higher  than  the 
preceding.  The  commercial  system  according  to  this  plan  employs 
60,  80  and  no  volts  in  the  order  named.  These  are  round  numbers 
that  are  practically  convenient,  but  are  only  approximately  in  the 
ratio  stated,  the  theoretically  correct  values  being  60,  79.5  and  105.3 
volts.  The  armature  voltages  and  speeds  obtained  by  connecting 
the  armature  terminals  of  the  typical  lo-horsepower  motor  to  the 
various  conductors  are  given  in  the  following  table.  The  speed  is 
proportional  to  c.e.m.f.;  that  is,  r.p.m.  =  (c.e.m.f.  -j-  218.2)  X  825, 
the  two  latter  being  rated  values. 

TABLE  VII.  — SPEED  CONTROL  OF  10-H.P.  MOTOR  BY  BULLOCK 
MULTIPLE-VOLTAGE  METHOD. 


Terminal    Volts. 

'«««+*>,. 

C.E.M.F. 

Speed  in  R.P.M. 

60 

11.8 

48.2 

182 

80 

11.8 

68.2 

258 

110 

11.8 

98.2 

374 

140 

11.8 

128.2 

485 

190 

11.8 

178.2 

673 

250 

11.8 

238.2 

900 

MULTIPLE-VOLTAGE   CONTROL   OF   MOTOR  SPEED. 


79 


The  Crocker- Wheeler  method  gives  a  speed  range  which  is  about 
i  to  8,  as  stated  above,  while  the  Bullock  arrangement  gives  a  speed 
range  of  exactly  i  to  5,  the  total  number  of  controller  steps  being 
the  same  for  both.  The  range  of  voltage  is  40  to  240  in  the  former 
and  50  to  250  in  the  latter.  Hence  the  former  starts  at  a  lower  speed 
of  106  r.p.m.  instead  of  182  r.p.m.,  and  finally  reaches  about  same 
maximum  of  862  compared  with  900  r.p.m. 

Multiple-voltage  systems  may  be  worked  at  any  reasonable  maxi- 
mum; they  differ  only  in  the  ratio  of  voltages.  It  is  desirable,  how- 
ever, to  have  standard  values  for  at  least  the  maximum  voltage  and 
one  of  the  sub-voltages,  in  order  that  standard  motors,  lamps,  etc., 
may  be  fed  from  the  lines. 

The  efficiency  o)  multiple-voltage  speed  control  is  much  higher 
than  that  of  the  armature  rheostat  method  for  same  torque  and  speed 
range.  Let  us  consider  the  typical  lo-h.p.  motor,  the  data  of 
which  were  given  in  the  table  on  p.  20,  and  determine  its  efficiency 
at  rated  torque  and  the  various  speeds  obtained  by  the  Crocker- 
Wheeler  multiple-voltage  system. 

The  various  speeds  at  rated  torque  corresponding  to  impressed 
voltages  of  40,  80,  120,  160,  200  and  240,  respectively,  are  deter- 
mined as  in  the  case  of  the  Bullock  system  above.  The  input  in  watts 
in  each  case  is  found  by  multiplying  the  voltage  input  by  the  rated 
armature  current  (Ia  =37  amperes)  and  adding  230  watts,  which  is 
the  normal  field  input  of  this  typical  lo-h.p.  motor. 


TABLE  VIII.— SPEEDS  AND  EFFICIENCIES  OF  10-H.  P.  MOTOR  WITH 
CROCKER- WHEELER  MULTIPLE-VOLTAGE  CONTROL. 


Output 

Volt- 
age 
Input  . 

Input. 
Watts. 

(laRa  +  Db) 

Volts. 

C.e.m.f. 
Volts. 

R.p.m. 

7570Xr.p.m.« 

Efficiency  at 
Rated  Torque. 

825 

Watts. 

40 

1710 

11.8 

28.2 

106. 

971 

56.6% 

80 

3190 

11.8 

68.2 

258. 

2370 

74.2 

120 

4670 

11.8 

108.2 

409. 

3750 

80.1 

160 

6150 

11.8 

148.2 

560. 

5150 

83.5 

200 

7630 

11.8 

188.2 

711. 

6530 

85.6 

240 

9110 

11.8 

228.2 

862. 

7920 

86.7 

Comparing  the  efficiency  curve  (Fig.  41)  of  the  multiple-voltage 
method  of  speed  control  with  that  of  the  armature  rheostat  method 


80        ELECTRIC   MOTORS,    THEIR   ACTION   AND    CONTROL. 

at  rated  torque,  Fig.  4,  the  much  higher  average  efficiency  of  the 
former  method  is  very  marked.  An  even  greater  advantage  of  this 
method  over  the  rheostatic  control  is  its  far  better  speed  regulation 
under  variable  loads.  The  curves  in  Fig.  42  and  values  in  the  fol- 
lowing table  show  this  superiority  very  clearly.  The  r.p.m.  at  rated 
torque  are  from  table  above,  and  r.p.m.  at  no  load  are  39  r.p.m. 


90 
80 
70 
60 

K 

1  50 

m 

|  40 

30 

20 

10 

0 


/  Efficiency 
Rheostatic  Co 


ntrol 


25  50    . 

Percent  Rated  Speed 


100 


FIG.    41.  —  COMPARATIVE  EFFICIENCIES   OF   RHEOSTATIC  AND  MULTIPLE -VOLT  AGE 

SYSTEMS. 


higher  in  each  case  for  multiple  voltage,  because  armature  current 
is  reduced  from  37  to  2.3  amperes,  which  decreases  armature  drop 
34.7  X  .28  =  9.7  volts,  and  brush  drop  is  .84  instead  of  1.4  volts. 
The  c.e.m.f.  must  rise  therefore  9.7  -f  .6  =  10.3  volts,  producing  a 
speed  increase  of  10.3  -r-  218.2  X  825  =  39  r.p.m.  The  r.p.m.  at 
no  load  for  armature  rheostat  control  are  found  as  follows:  The 
terminal  pressure  to  give  106  r.p.m.  at  rated  torque  is  40  volts; 
hence  240  —  40  =  200  volts  must  be  consumed  in  rheostat,  the 
resistance  of  which  is  200  -r-  37  =  5.4  ohms.  At  no  load  c.e.m.f. 
=  240  -  23  (.28  +  5.4)  -  .84  =  226.1  volts.  This  corresponds  to 
226.1  -r-  218.2  X  285  =  854  r.p.m.  as  given  in  Table  IX. 


MULTIPLE-VOLTAGE   CONTROL   OF   MOTOR  SPEED.  81 

TABLE  IX.  — SPEED  REGULATION,  10-H.  P.  MOTOR. 


Multi-  Voltage  Control,  Vmax=  240  volts. 

Rheostatic  Control,  VmM  =  240  volts. 

R.p.m. 
Rated  Torque. 

R.p.m. 
No  Load. 

Per  Ct. 
Speed 
Change. 

R.p.m. 
Rated  Torque. 

R.p.m. 
No  Load. 

Per  Ct. 

Speed 
Change. 

Curve  A  106 

145 

13.7 

Curve  a  106 

854 

705 

Curve  B  258 

296 

11.5 

Curve  b  258 

865 

235 

Curve  C  409 

447 

9.2 

Curve  c  409 

875 

114 

Curve  D  560 

599 

7.0 

Curve  d  560 

883 

48.4 

Curve  E  711 

750 

5.2 

Curve  e  711 

892 

25.2 

Curve  F  862 

900 

4.6 

Curve  F  862 

900 

4.6 

A  direct  and  really  fair  comparison  of  efficiency  between  the  field- 
control  methods  and  the  multiple-voltage  control  is  impossible,  since 
the  motor  with  full  rated  field  and  armature  currents  runs  at  mini- 
mum speed  and  at  or  near  its  maximum  efficiency  in  the  former  case 


0  2f>  50 

Percent  Rated  Load 
FIG.   42.  —  SPEED   REGULATION   OF   RHEOSTATIC   AND   MULTIPLE- VOLTAGE 

SYSTEMS. 

contrasted  with  maximum  speed  and  efficiency  in  the  latter  case. 
The  former  attains  the  higher  speeds  by  field  weakening,  while  the 
latter  obtains  the  lower  speeds  by  decreased  voltage  applied  to  the 
armature.  Furthermore,  the  first  gives  a  constant  output  in  horse- 


82        ELECTRIC  MOTORS,    THEIR   ACTION  AND   CONTROL. 


power,  the  torque  varying  inversely  with  the  speed;  whereas  the 
second  can  give  full  torque  at  all  speeds,  so  that  the  horsepower  out- 
put obtainable  varies  directly  with  the  speed.  Hence  the  two 
methods  call  for  different  types  of  motors  and  cannot  properly  be 
considered  on  the  same  basis. 

A  multiple-voltage  system  can  be  supplied  by  three  generators 
of  40,  80  and  120  volts  respectively,  each  large  enough  to  carry  its 
corresponding  fraction  of  the  maximum  load.  This  does  not,  how- 
ever, necessarily  equal  the  combined  watt  capacity  of  the  motors,  as 
it  is  improbable  that  all  machines  will  be  simultaneously  operating  at 
full  output.  In  fact  the  actual  working  load  is  not  likely  to  exceed 
30  to  50  per  cent  of  the  possible  load.  Instead  of  using  the  above 
combination  of  three  generators,  one  generator  of  total  voltage  and 
load  capacity  and  a  three-unit  balancing  set  (Fig.  43)  of  40,  80  and 


121 


SO  V. 


FIG.    43.  —  MULTIPLE-VOLTAGE  SYSTEM  WITH  BALANCERS. 

120  volts  can  be  and  usually  is  employed.  These  balancers,  it  has 
been  found  by  experience,  need  have  a  total  capacity  of  only  5  to  10 
per  cent  of  the  total  load  in  ordinary  cases.  If,  however,  there  is 
one  extremely  large  motor,  while  the  rest  of  the  plant  consists  only 
of  small  motors,  the  balancer  set  should  have  a  capacity  equal  to 
that  of  this  large  motor.  The  balancer  arrangement  is  the  one 
usually  adopted,  as  it  is  advantageous  in  the  following  respects: 

(a)  Lower  cost  of  prime  movers,  only  one  instead  of  either  three 

engines  or  a  system  of  line  shafts,  belts,  etc. 

(b)  Lower  cost  of  generator,  a  large  one  in  place  of  three  smaller 

ones  of  same  aggregate  power. 

(c)  Lower  cost  of  foundations. 

(d)  Less  steam  piping. 

(e)  Cheaper  switchboard  and  electrical  connections. 

The  motors  controlled  by  multiple  voltage  are  ordinary  standard 


MULTIPLE-VOLTAGE   CONTROL   OF   MOTOR   SPEED. 


83 


machines,  which  is  an  important  practical  advantage.  They  are  so 
connected  that  the  field  is  permanently  across  the  24o-volt  lines 
whenever  the  motor  is  in  operation,  and  the  six  running  speeds  are 
obtained  by  shifting  the  armature  terminals  from  sub-voltage  to  sub- 
voltage  by  means  of  the  controller  drum  as  shown  in  Fig.  44.  In 


FIG.  44.  —  CONTROLLER  AND  MOTOR  CONNECTIONS,  MULTIPLE-VOLTAGE  SYSTEM. 

some  cases  the  changes  from  one  running  speed  to  another  are  made 
gradually  by  shifting  to  the  next  higher  voltage  with  some  resistance 
inserted  in  the  armature  circuit  and  then  gradually  reducing  this 
resistance  until  that  voltage  is  applied  to  the  armature  terminals  and 
so  on  with  the  various  sub-voltages  until  the  maximum  pressure  is 
attained.  These  gradual  changes  with  intermediate  speeds  are  also 
obtainable  by  diminishing  the  field  current  until  the  next  higher 
speed  is  reached,  then  connecting  the  armature  to  the  corresponding 
voltage,  at  the  same  time  reestablishing  full  field  current.  Since  the 
speed  steps  differ  by  only  10  or  15  per  cent,  the  ordinary  shunt  motor 
is  capable  of  this  range  of  field  weakening. 

In  some  instances  a  combination  of  variable  field  current  and 
armature  rheostat  control  is  employed  in  passing  from  one  sub-volt- 
age to  another,  thus  obtaining  as  many  as  36  different  speed  points 
from  minimum  to  maximum. 


84        ELECTRIC  MOTORS,    THEIR   ACTION   AND   CONTROL. 

The  Motor-Generator  and  "  Boost  and  Retard  "  Systems,  both 
invented  by  H.  Ward  Leonard,  are  also  multi-voltage  or  rather  ad- 
justable voltage  methods  of  speed  control,  but  the  speed  changes 
being  gradual,  no  intermediate  steps  are  required.  In  the  case  of 
the  motor-generator  system*  in  addition  to  the  working  motor,  a 
motor-dynamo  is  required  for  each  machine  so  operated.  The 
motor  end  (M)  of  the  motor-dynamo  is  connected  to  the  line  or 
supply  mains  and  controlled  as  any  ordinary  single-speed  machine 
(Fig.  45) .  The  generator  terminals  (D)  are  connected  to  the  work- 


no.    45.  —  WARD  LEONARD  MOTOR-GENERATOR  SYSTEM  OF  CONTROL. 

ing  motor's  armature  (WM).  Adjustable  voltages  and  speeds 
are  obtained  by  changes  in  the  field  strength  of  the  generator,  the 
field  of  which,  as  well  as  that  of  the  working  motor,  being  connected 
to  the  supply  circuit.  Reversal  of  rotation  in  this  case  is  by  means 
of  a  reversing  switch  (5)  and  rheostat  (R)  in  the  generator  field  cir- 
cuit. By  this  method  the  reversal  of  voltage  applied  to  the  working 
motor  is  gradually  accomplished,  being  first  reduced  to  zero  and  then 
built  up  in  the  opposite  direction.  Furthermore  the  reversing  switch 
5  controls  only  a  small  field  current  instead  of  the  armature  current 
which  would  be  20  to  50  times  greater,  requiring  large  contact 
surfaces. 

While  this  system  is  extremely  flexible,  it  is  not  extensively  em- 
ployed on  account  of  its  great  cost.  The  motor  end  of  the  motor- 
dynamo  must  be  larger  than  the  working  motor  by  the  amount  of  the 
losses  in  both  the  dynamo  and  working  motor.  For  example,  the 
lo-horsepower  motor  previously  considered  is  of  86.6  per  cent  effi- 
ciency; hence  to  operate  this  machine  at  rated  load,  the  input  must 
be  10  -T-  .866,  or  n.6  horsepower.  The  efficiency  of  the  dynamo  is 
also  about  the  same,  so  the  motor  end  of  the  motor-dynamo  must  be 

*  U.  S.  Patent  No.  463,802,  November,  1891. 


MULTIPLE-VOLTAGE   CONTROL   OF   MOTOR  SPEED.  85 

of  1 1.6  -j-  .866,  or  13.5  horsepower  capacity.  Thus  three  machines 
are  required,  each  of  a  power  equal  to  or  somewhat  greater  than  that 
needed  for  the  actual  work,  the  total  rated  capacity  being  10  + 
ii. 6  +  13.5  =  35.1  horsepower. 

The  most  extensive  use  of  this  method  of  speed  control  is  for  the 
operation  of  turrets  and  gun  platforms  in  modern  war-ships,  for 
which  very  fine  adjustment  and  yet  wide  range  in  speed  are  neces- 
sary. It  is  now  used  also  for  driving  large  rolls  in  steel  mills, 
where  in  combination  with  a  heavy  flywheel  it  is  known  as  the  Ilgner 
system. 

Both  of  the  Ward  Leonard  methods  and  the  "teaser"  system,  to 
be  given  later,  involve  motor-generator  equipments.  Their  essen- 
tial advantage  is  forcibly  shown  by  the  following  example :  To  obtain 
55.5  amperes  at  17  volts,  sufficient  to  develop  a  torque  to  start  the 
standard  lo-horsepower  motor  from  rest  under  load,  assuming  50 
per  cent  increase  in  armature  current  above  the  rated  value  of  37 
amperes,  would  require  a  motor-generator  of  80  per  cent  efficiency 
to  draw  55.5  X  17  -=-  .8  =  944  watts  from  the  line,  whereas  to  obtain 
the  same  starting  torque  directly  from  a  23<D-volt  line  by  means  of 
armature  rheostat  control  would  require  55.5  X  230  or  12.77  kilo- 
watts, which  is  nearly  14  times  as  much  power. 

The  "Boost  and  Retard'1  System*  of  Ward  Leonard  is  very  similar 
in  principle  to  the  preceding  method  but  reduces  somewhat  its 
high  cost  by  the  following  scheme.  A  motor-generator  is  also  em- 


FIG.   46.  — WARD  LEONARD  "BOOST  AND  RETARD"  SYSTEM  OF  CONTROL. 

ployed,  but  the  generator  end  is  placed  in  series  with  the  line,  so  that 
its  e.m.f.  may  be  added  to  or  opposed  to  the  line  pressure.  This  e.m.f. 
is  controlled  by  variation  of  field  resistance,  and  its  direction  by  rever- 
sal of  field  connections.  The  operation  of  this  system  may  be  under- 
stood by  referring  to  Fig.  46.  To  obtain  the  same  speed  range  with 

*  U.  S.  Patent  No.  572,903,  December  8,  1896. 


86         ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

a  24o-volt  motor  as  with  an  ordinary  multiple-voltage  system,  the 
line  potential  is  only  120  volts,  while  the  generator  end  D  also 
develops  120  volts.  Thus  if  both  line  and  generator  pressures  are 
in  series,  the  voltage  V  at  the  motor  terminals  will  be  120  +  120  or 
240.  Decrease  in  value  of  V  from  240  volts  to  120  volts  is  obtained 
by  weakening  the  field  of  the  generator  to  zero,  while  a  reduction  of 
V  below  120  volts  is  obtained  by  reversing  the  generator  voltage  and 
thus  subtracting  it  from  the  line  pressure.  This  is  accomplished  by 
arranging  the  field  rheostat  F  to  reverse  the  field  connections,  the 
current  in  the  same  having  been,  however,  first  gradually  reduced 
practically  to  zero,  in  which  manner  the  high  voltage  and  spark 
accompanying  the  opening  of  a  field  circuit  are  eliminated.  Since 
the  voltage  of  the  generator  end  is  only  one-half  of  that  required  by 
the  working  motor  WM  at  rated  speed,  the  watt-capacity  of  the 
"boost  and  retard"  equipment  need  be  only  a  little  more  than  one- 
half  that  of  the  working  motor,  and  accordingly  the  parts  of  the 
motor  generator  MD  are  50  per  cent  smaller  than  in  the  preceding 
system. 

The  voltage  and  current  relations  existing  between  the  units  com- 
prising the  "boost  and  retard"  system  are  as  shown  in  Table  X, 
and  it  should  be  noted  that  when  the  generator  end  of  the  MD 
set  is  "crushing"  or  "retarding"  the  line  voltage,  it  has  reversed 
its  function  and  is  acting  as  a  motor  driving  what  was  previously 
the  motor  end  as  a  generator,  which  then  pumps  back  into  the  sup- 
ply line,  thus  furnishing  part  of  the  current  required  by  the  work- 
ing motor.  For  example,  to  run  the  working  motor  at  one-quarter 
speed  or  206  r.p.m.  requires  n.8  +  (218.2  -5-  4)  =  66.3  volts,  that 
is,  armature  and  brush  drop  plus  one-quarter  of  rated  c.e.m.f. 
Hence  the  machine  D  must  generate  —  48.7,  which,  combined  with 
115,  the  line  voltage,  produces  the  required  66.3  volts  for  the  working 
motor.  The  machine  Z>,  thus  developing  a  c.e.m.f.  of  —  48.7  volts,  is 
therefore  running  as  a  motor,  consuming  48.7  X  37  =  1802  watts. 
Assuming  the  combined  efficiency  of  the  machines  D  and  M  as  80 
per  cent,  the  latter  will  generate  .80  X  1802  =  1442  watts  at  115 
volts,  .since  it  is  connected  to  the  supply  lines.  Hence  it  furnishes 
1442  -T-  115  =  12.6  amperes  and  the  supply  circuit  24.4  amperes  to 
make  up  the  37  amperes  consumed  by  the  working  motor.  The 
other  values  in  Table  X  are  calculated  in  a  similar  manner.  For 
small  currents  the  efficiency  of  D  and  M  might  be  less  than  80  per 


MULTIPLE-VOLTAGE   CONTROL   OF  MOTOR  SPEED. 


87 


cent,  but  the  difference  would  be  of  little  practical  consequence.  It 
is  to  be  noted  that  this  "boost  and  retard"  method  as  well  as  the 
preceding  "  motor-generator "  arrangement  gives  full  rated  torque 
with  usual  overload  capacity  at  all  speeds  of  the  working  motor,  so 
that  its  horsepower  output  increases  directly  with  its  speed. 

TABLE  X.— BOOST  AND  RETARD  EQUIPMENT  —  VOLTAGE  AND 
CURRENT  RELATIONS. 


Working    Motor. 

Motor-Dynamo. 

Supply  Line. 

Dynamo  End. 

Motor  End. 

Speed 
R.p.m. 

Volts. 

Amp. 

Volts. 

Amp. 

Volts. 

Amp. 

#olts. 

Amp. 

0 

11.8 

37 

-103.2 

+  37 

115 

26.5 

115 

10.5 

206 

66.3 

37 

-48.7 

37 

115 

12.6 

115 

24.4 

412 

120.8 

37 

5.8 

37 

115 

-   2.3 

115 

39.3 

618 

175.3 

37 

60.3 

37 

115 

-23 

115 

60.0 

825 

230.0 

37 

115.0 

37 

115 

-46.8 

115 

83.8 

Examination  of  the  table  shows  that  this  system  of  control  is 
advantageous  at  speeds  considerably  below  the  rated  value.  For 
example,  to  start  the  working  armature  by  supplying  it  with  n.8 
volts  consumes  only  115  volts  and  10.5  amperes  or  1208  watts  from 
the  supply  lines.  To  produce  the  same  effect  with  a  rheostat  in  the 
armature  circuit  would  demand  230  volts  and  37  amperes  or  8510 
watts,  which  is  seven  times  as  large  an  input.  On  the  other  hand, 
with  this  system  at  or  near  rated  speed,  the  efficiency  falls  from  86.7 

per  cent  for  the  individual  motor  to  — =  71.6  per  cent  for 

115  X  83.8 

the  combination  of  the  motor  and  motor-generator. 

Bullock  "  Teaser  "  System.  —  This  arrangement  is  designed  espe- 
cially for  printing-press  operation  when  the  " inching"  or  slight 
forward  movement  of  the  press  must  be  effected  very  accurately  for 
"  making  ready, "  as  it  is  Called.  While  this  could  be  accomplished 
by  the  two  preceding  multi-voltage  methods,  the  cost  of  the  equip- 
ment would  be  rather  high,  hence  the  development  of  this  special 
method. 

The  apparatus  and  connections  of  the  electrical  units  are  as  shown 
in  Fig.  47.  The  "teaser"  or  motor-generator  MD  is  of  compara- 
tively small  capacity  and  its  generator  end  generates  a  low  voltage. 


88         ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 


The  operation  is  as  follows:  The  motor-generator  acts  as  a  current 
transformer,  to  supply  currents  of  considerable  value  at  low  voltage 
to  the  main  motor  for  starting  large  presses,  inching  them  forward 
or  even  running  them  for  long  periods  at  very  low  speeds.  The 
speed  of  the  working  motor  WM  is  gradually  augmented  by  increas- 


Teasev  Set 


Teaser  Set  Cut-out 

Working  Motor  connected 

directly  to  Line 


Full  Speed 
FIG.    47.  —  BULLOCK   " TEASER"   SYSTEM. 


ison  — 

12S   l\ 

Hv 

orking 
•  Line  < 
Rheos 

Motor 
Barren 
at  Cor 

Currer 
-with 
trol 

t 
Teasd 

r  Cut  ( 

)ut 

D 

InU 

/ 

/ 

T 

<U    K/\L|  

LineC 

'urrent 

with! 

easer  S 

it 

x 

/ 

x 

0        10       20       30        40        50        60        70       80       90     10( 

Percent  Rated  Speed 

FIG.  48.  —  LINE  CURRENTS,  RHEOSTATIC  AND  "TEASER"  CONTROL. 

ing  the  speed  and  voltage  of  D  by  decreasing  the  value  of  the  series 
resistance  R  or  by  field  weakening  of  the  motor  end  until  the  main 
motor  WM  is  rotating  at  such  a  rate  that  it  can  be  operated  with 
comparative  economy  from  the  main  line  through  the  resistance  Rm, 
at  which  instant  the  "teaser"  is  disconnected  from  the  line  and  main 


MULTIPLE-VOLTAGE  CONTROL  OF  MOTOR  SPEED.  89 

motor.  The  great  economy  of  the  teaser  system  over  armature 
rheostat  control  is  proved  by  the  curves  in  Fig.  48,  which  represent 
the  currents  drawn  from  the  line  by  the  two  methods  when  perform- 
ing the  same  duty  with  the  working  motor.  The  dotted  line  com- 
pared with  the  solid  line  shows  the  reduction  in  line  current  with  the 
teaser,  the  saving  being  more  than  50  per  cent  up  to  about  30  per 
cent  of  rated  speed.  A  little  below  half  speed  the  teaser  is  cut  out. 

The  conditions  while  starting  the  typical  lo-horsepower  motor  are 
represented  in  the  upper  diagram  of  Fig.  47.  The  armature  current 
of  the  working  motor  WM  is  assumed  to  be  55.5  amperes,  which  is 
50  per  cent  above  rated  value,  in  order  to  overcome  inertia  and 
initial  friction.  Of  this  current  48.8  amperes  are  generated  by  the 
generator  end  D  of  the  teaser,  and  6.7  amperes  are  supplied  through 
the  motor,  as  indicated.  Merely  to  start  the  motor  demands  17 
volts  and  48.8  amperes  or  830  watts  from  the  machine  D.  Assum- 
ing 80  per  cent  efficiency  for  the  motor-generator,  the  input  of  the 
motor  end  M  must  be  830  -5-  .8  =  1036  watts.  Hence  the  voltage 
consumed  by  it  is  1036  -f-  6.7  =  150  volts  and  the  drop  in  the  series 
resistance  R  is  230  -  (150  +  17)  =63  volts,  the  amount  of  this 
resistance  being  63  -=-  6.7  =  9.4  ohms,  which  is  gradually  decreased 
to  raise  the  speed  of  the  teaser  and  working  motor.  At  starting  only 
230  volts  and  6.7  amperes  are  drawn  from  the  supply  lines,  instead 
of  230  volts  and  55  amperes,  which  is  more  than  eight  times  the 
power  in  watts. 

When  the  teaser  generator  is  of  the  simple  shunt  type,  a  sudden 
overload  or  sticking  of  the  press  rollers  stalls  the  entire  equipment, 
because  the  terminal  volts  of  D  fall  too  low  to  produce  the  current 
required.  To  overcome  this  difficulty  the  modification  known  as 
the  Bullock  Teaser  Booster  equipment  has  been  developed.  This  is 
essentially  like  the  preceding,  but  the  generator  end  of  the  teaser  is 
compound  wound,  so  that  any  tendency  to  stall  the  working  motor 
WM  increases  the  current;  thus  the  voltage  of  D  and  the  motor 
torque  are  sufficiently  augmented  to  carry  it  over  the  sticking  point. 
With  these  teaser  arrangements,  the  working  motor  may  exert  full 
torque  at  all  speeds,  so  that  its  horsepower  output  increases  with  the 
latter  as  in  the  multiple- voltage  or  "boost  and  retard"  systems. 

Holmes-Clatworthy  System.  —  This  is  similar  to  the  teaser  sys- 
tem in  principle  and  is  also  applicable  to  the  driving  of  printing 
presses,  but  the  low  speed  for  starting  and  inching  purposes  is 


90         ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

supplied  from  a  special  motor.  The  equipment  comprises  a  main 
or  working  motor,  a  smaller  auxiliary  motor  and  a  controller. 
In  addition  there  is  an  electrically  operated  self-releasing  clutch, 
situated  between  the  two  motors,  by  which  the  turning  effort  of 
the  auxiliary  motor  is  transmitted  through  worm  gears  to  the  press. 
The  auxiliary  motor  is  wound  for  such  a  speed  that  by  means  of 
the  gearing  it  will  drive  the  press  for  all  purposes  of  starting  up, 
inching,  leading  in,  etc.,  and  run  it  up  to  a  sufficient  speed  so  that 
the  main  motor  may  take  the  load  advantageously.  As  soon  as  the 
main  motor  overspeeds  the  auxiliary  one,  the  self-releasing  clutch  is 
automatically  operated,  and  the  latter  machine  is  disconnected. 

Double-armature  Method.  —  This  method  of  motor  speed  con- 
trol is  placed  under  the  general  head  of  multi-voltage  or  ad- 
justable-voltage systems  because  even  though  the  line  voltage 
remains  constant,  adjustable  speed  is  obtained  by  changing  the 
voltage  applied  to  a  given  armature  winding,  thus  producing 
the  same  result  as  by  altering  line  voltage.  There  are  two  gen- 
eral arrangements  belonging  to  this  class.  The  principle  is  the 
same  for  both,  but  with  the  first  only  two  running  speeds  are 
obtained  by  connecting  the  armature  windings  either  independently 
or  in  series,  while  in  the  second  four  speeds  can  be  secured  by 
changes  in  the  manner  of  connecting  the  two  armature  windings  to 
the  circuit. 

The  first  method  (General  Electric  Company's,*  and  C.  and  C. 
Electric  Company's  systems)  employs  a  motor  with  an  ordinary  field 
frame  and  winding  which  may  be  shunt  or  may  be  compound  wound, 
but  the  armature  core  is  provided  with  two  windings  and  two  com- 
mutators, which  are  alike  in  all  respects.  Thus  if  one  armature 
winding  be  placed  across  the  line  a  certain  speed  will  be  obtained. 
If  both  are  placed  across  the  line  in  series,  the  speed  will  be  about 
one-half  as  great.  This  double-armature  method  is  closely  similar  to 
the  series-parallel  control  of  railway  motors.  The  successive  steps 
in  this  method  for  the  speed  regulation  of  a  compound-wound 
motor  are  as  illustrated  in  Fig.  49. 

An  extension  of  the  same  principle  is  exemplified  in  the  motor 
developed  by  the  Commercial  Electric  Company,  which  employs 
one  common  field  frame  and  winding  (shunt  or  compound)  and  two 
independent  armature  windings,  but  instead  of  having  these  alike  in 

*  U.  S.  Patent  No.  757,394,  April,  1904. 


MULTIPLE-VOLTAGE   CONTROL  OF   MOTOR  SPEED. 


91 


number  of  inductors,  one  of  them  has  2  x  inductors  and  the  other  3  x 
inductors;  i.e.,  one  has  50  per  cent  more  inductors  in  series  than  the 
other.  Thus  if  the  2  x  winding  be  opposed  to  the  3  x  winding  and 
connected  in  series  to  the  line,  only  x  inductors  are  effective  in  pro- 
ducing the  c.e.m.f.,  hence  the  speed  would  be  a  maximum.  If  the 


Min.  Speed 


Shunt  Field 


Series  Field 


Armature  Windings 


Starting 

Resistance 


TfiTtfTORRr 


4- 

f-\ 


ymrwtf 


Series  Field  Jumper 

Max  Speed 


Shunt  Field 


FIG.  49.  —  GENERAL  ELECTRIC,  DOUBLE-ARMATURE  MOTOR  CONTROL. 

winding  with  2  x  inductors  were  connected  by  itself  to  the  line,  a 
speed  of  one-half  the  maximum  would  be  obtained.  If  the  winding 
with  3  x  inductors  were  placed  across  the  line,  a  speed  of  one-third  the 
maximum  would  be  obtained,  while  if  both  were  placed  in  series 
across  the  line  so  that  they  generate  e.m.f.  in  the  same  direction 
corresponding  to  5  x  inductors,  a  speed  of  only  one-fifth  the  maximum 
would  be  the  result.  The  general  connections  for  these  steps  are 
shown  in  Fig.  50.  If  used  in  combination  with  field  or  with  rheo- 


92 


ELECTRIC   MOTORS,    THEIR   ACTION   AND    CONTROL. 


static  control  this  method  would  give  an  extremely  wide  range;  for 
example,  a  6  to  i  field  range  would  give  a  30  to  i  speed  range. 

The  series-parallel  control,  including  ordinary  railway  motors,  as 
well  as  the  G.  E.  and  C.  and  C.  methods  described  above,  gives  full 
rated  torque  at  all  speeds  unless  the  field  is  weakened,  because  both 


Min.  Speed 


+1 

3z 

11 

YL 

rTj 

2x 

^EEEE 

2X  turns 

Y 

B 

Max.  Speed 


*'     .Winding  |J_L 
3x 
—  T     Turns     1  .    . 

JQ_|  Winding 
2X 
•1  Turns 

50T 

sx-aa 

r.  p.  m. 
=1X  turns 

FIG.    50.  —  COMMERCIAL  ELECTRIC  COMPANY'S  DOUBLE-ARMATURE  METHOD  OF 

MOTOR    CONTROL. 


armatures  may  carry  full  current,  if  desired.  The  Commercial  Com- 
pany's arrangement  exerts  only  one-fifth  torque  at  five  times  the 
speed,  that  is,  constant  power  like  field  weakening. 

Speed  Control  by  Variation  of  Number  of  Poles.  — The  Bullock 
Company  at  one  time  manufactured  a  motor  capable  of  giving 
various  speeds  by  changing  the  number  of  poles.  For  example, 
consider  a  six-circuit  armature  with  a  six-pole  field  magnet.  When 
the  field  coils  are  so  connected  that  the  ordinary  relation  of  alternate 
north  and  south  polarity  exists,  the  number  of  armature  circuits 


MULTIPLE-VOLTAGE   CONTROL   OF   MOTOR  SPEED.  93 

(b)  and  the  number  of  poles  (2  p)  are  each  6,  hence  from  equation  (2), 
the  speed  would  be 

eios6o  b       e  io86o 

r.p.m.  =   — =  - 

n<&  2  p  n$ 

If,  then,  the  connection  of  the  field  coils  be  changed  so  that  three 
poles  adjoining  each  other  become  S,  and  the  other  three  N,  we  have 

e  io86o 

with  the  same  impressed  voltage  a  speed  in  r.p.m.  =  -      —  or  J# 

»3$ 

because  the  armature  winding  becomes  a  two-circuit  one,  the  number 
of  poles  being  two,  while  the  flux  per  pole  has  increased  to  about  3  </> 
provided  the  yoke  and  armature  have  sufficient  cross  section  to 
carry  the  increased  flux.  Hence  the  speed  in  the  second  case  would 
be  only  one-third  of  what  it  was  in  the  first  instance.  The  cost  of 
this  design  is  so  great,  due  to  larger  frame,  complex  windings  and 
switches,  that  it  has  not  been  commercially  successful.  For  example, 
it  would  be  necessary  even  in  the  smallest  motors  to  have  six  poles  in 
order  to  obtain  a  3  to  i  speed  range. 

For  further  discussion  of  these  various  systems  of  shunt-motor  control  see  the 
following  publications: 

D.  C.  MOTOR  SPEED  REGULATION.     J.  W.  Rogers.     Prac.  Eng.,  London,  1907. 

DEE  GLEICHSTROMMASCHINE.     E.  Arnold.     Vol.  II,  p.  616,  1908. 

ELECTRIC  JOURNAL,  Vol.  I,  p.  251;  Vol.  II,  pp.  u,  566;  Vol.  Ill,  p.  348. 

ELECTRIC  MOTORS.     H.  M.  Hobart.     1904. 

ELECTRIC  WORLD,  Vol.  XLIX,  p.  947. 

ENGINEERING,  September,  1905. 

LONDON  ELECT.,  Jannary  27,  1905. 

MOTOR  CONTROL.  American  Electrician,  Vol.  XVI,  1904,  p.  391;  Vol.  XVII, 
1905,  p.  303. 

MULTIPLE- VOLTAGE  CONTROL,  Electric  Power,  1904. 

PROCEEDINGS  ENG.  SOCIETY  WESTERN  PA.,  October,  1905. 

SPEED  CHARACTERISTICS  AND  CONTROL  OF  ELECTRIC  MOTORS.  C.  F.  Scott.  Eng. 
Mag.,  Vol.  XXXI,  p.  60,  1906. 

TRANSACTIONS  A.  I.  E.  E.,  Vol.  XIII,  p.  377,  1896;  Vol.  XX,  pp.  111-197,  !902- 

ELECTRIC  MOTORS  IN  MACHINE  SHOP  SERVICE.  Chas.  Day.  Trans.  Internat. 
Elect.  Congress,  Vol.  I,  1904,  p.  591. 

VARIABLE-SPEED  CONTROL.    Eng.  U.  S.  A.,  1904. 

PUBLICATIONS  CROCKER- WHEELER,  BULLOCK,  ELECTRO-DYNAMIC,  GENERAL  ELEC- 
TRIC AND  WESTINGHOUSE  COMPANIES. 


CHAPTER    IX. 

DIRECT-CURRENT   SERIES    MOTORS. 

As  the  name  of  this  motor  implies,  the  field  and  armature  windings 
are  in  series,  Fig.  51,  hence  the  same  current  that  flows  through 
the  armature  also  excites  the  field  magnet. 


FIG.   51.  CONNECTIONS  OF  SERIES  MOTOR. 

Series  motors  are  of  two  general  types,  constant  potential  and 
constant  current.  Attention  will  be  paid  chiefly  to  the  former,  since 
the  latter  are  no  longer  used  commercially.  The  greatest  of  all 
applications  of  the  electric  motor  is  to  electric  traction,  for  which 
purpose  series  motors  are  almost  universally  used  in  this  country.  It 
is  natural,  therefore,  to  discuss  the  action  and  control  of  series 
motors  from  the  railway  standpoint,  although  their  application  to 
hoists,  fans,  pumps,  etc.,  is  also  important. 

Speed-current  Curve.  —  The  speed  of  a  series  motor  rises  when 
the  current  is  diminished,  the  exact  relation  depending  upon  the 
degree  of  magnetization  of  the  magnetic  circuit.  At  low  current 
values,  and  therefore  low  flux  densities,  the  speed  is  relatively  high 
and  the  amount  of  its  variation  for  a  given  change  in  torque  is  cor- 
respondingly great.  The  speed  is  much  lower  and  more  nearly 
constant  when  the  field  approaches  saturation.  The  general  formula 
for  the  speed  of  a  motor  as  already  given  in  equation  (2)  is 

E  io86o  b 
r.p.m.  = 

94 


DIRECT-CURRENT  SERIES  MOTORS.  95 

The  flux  4>  is  the  only  variable  and  depends  upon  the  field  current, 
in  the  case  of  a  series  motor,  being  the  same  as  or  proportional  to  the 
armature  current.  The  flux  at  low  densities  increases  almost  di- 
rectly with  the  current,  and  if  there  were  no  voltage  drop  due  to 
resistance,  the  speed  current  curve  would  take  the  form  r.p.m.  X  <3>  = 
Constant,  which  is  an  equilateral  hyperbola,  asymtotic  to  both  coor- 
dinate axes.  However,  as  saturation  of  the  magnetic  circuit  is  ap- 
proached there  is  a  gradual  reduction  in  the  rate  of  increase  of 
flux  with  current,  so  that  the  speed  does  not  fall  as  rapidly,  thus 
raising  the  right  hand  portion  of  the  curve.  Moreover,  the  resist- 
ance drops  of  the  armature  and  field  windings  increase  with  the 
current,  tending  also  to  raise  the  same  part  of  the  curve.  This 
relation  between  speed  and  amperes  input  is  brought  out  numeri- 
cally in  the  two  following  examples.  The  first  assumes  a  series 
motor  A  with  a  field  of  relatively  low  flux  density,  and  the  sec- 
ond a  series  motor  B  of  equal  current  capacity  but  having  a  field 
approaching  saturation  below  rated  load. 

The  series  motor  A  is  assumed  to  run  on  a  5 50- volt  constant- 
potential  circuit,  its  armature  resistance  being  0.7  ohm  and  field 
resistance  of  the  same"  value.  This  motor,  operated  as  a  dynamo 
(separately  excited)  at  a  constant  speed  of  200  r.p.m.,  gives,  in  terms 
of  voltage  generated,  the  magnetization  curve  A  in  Fig.  52  with  field- 
current  variations  from  o  to  50  amperes.  Armature  reaction  may  be 
practically  neglected,  being  relatively  small  in  series  machines  since 
the  brushes  are  in  the  neutral  position  and  because  field  m.m.f.  rises 
with  armature  current  and  m.m.f.  Brush  drop  is  also  a  practically 
negligible  item  in  most  series  motors  which  run  at  550  volts  or  more  in 
railway  service  and  usually  at  voltages  of  220  or  higher  for  stationary 
work.  Moreover,  the  field  winding  being  in  series  with  the  armature, 
drop  due  to  resistance  is  about  twice  as  great  as  in  a  shunt  machine 
of  the  same  voltage,  making  brush  drop  relatively  small,  and  it  will 
be  considered  as  included  with  the  armature  drop.  (See  Chapter  III, 
pp.  16-19.) 

The  speed-current  curve  is  calculated  as  follows: 

At  five  amperes  input  the  voltage  drop  due  to  armature  and  field 
resistance  is  IRa  +  IRse  =  5  (.7  +  .7)  =  7  volts;  hence  the  c.e.m.f. 
generated  with  550  volts  applied  =  550  —  7  =  543  volts.  From 
Fig.  5 2, curve  A,  the  field  flux  due  to  5  amperes  produces  at  200  r.p.m. 
an  e.m.f.  of  56.5  volts;  hence  to  develop  a  c.e.m.f.  of  543  volts  the 


96 


ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 


500 
450 
400 

350 
300 


§ 
3200 

150 
100 
50 


A 


4 


0         5         10         15        20        25        30        35        40        45        50 
Magnetizing  Current 

FIG.    52.  MAGNETIZATION  CURVES  OF  SERIES  MOTORS,  A  AND  B. 

r.p.m.  =  (543  -h  56.5;  X  200  =  1920.  The  speed  at  other  torque 
conditions  corresponding  to  10,  20,  30,  40  and  50  amperes  can  be 
similarly  calculated;  the  results  being  given  in  the  following  table. 


TABLE   XI.  —  CURRENT-SPEED    DATA,   SERIES    MOTOR    A    (LOW 
FLUX  DENSITY). 


Amp. 

A 

B 

C 

C.e.m.f.= 
A-CB+C) 

Volts  at  200  r.p.m. 
Curve  A,  Fig.  52. 

200Xc.e.m.f. 

V. 

'*. 

IRM 

Volts  at  200  r.p.m. 

5 

550 

3.5 

3.5 

543 

56.5 

200    (543-=-   56.  5)=  1920 

10 

550 

7.0 

7.0 

536 

108.0 

200    (536^108     )=1155 

20 

550 

14.0 

14.0 

522 

200.0 

200    (522-T-200     )=   522 

30 

550 

21.0 

21.0 

508 

283.0 

200   (508-=-  283     )=   359 

40 

550 

28.0 

28.0 

494 

350.0 

200    (494-=-  350     )=   283 

50 

550 

35.0 

35.0 

480 

412.0 

200    (480-^412     )=   233 

Plotting  these  speed  and  current  values  in  the  form  of  a  curve,  a, 
Fig.  53,  and  comparing  the  various  values,  it  is  seen  that  the  speed 
varies  greatly  with  the  current,  the  range  being  from  1920  to  233 
r.p.m.  with  currents  from  5  to  50  amperes. 


DIRECT-CURRENT  SERIES  MOTORS. 


97 


In  the  case  of  series  motor  B,  the  armature  and  field 
0.4  and  i.o  ohm  respectively,  and  the  magnetization- 
of  this  machine  operating  at  200  r.p.m.  with  5  to  50 
current  is  curve  B,  Fig.  52,  showing  much  higher  flux 
curve  A  of  the  first  machine.     The  rated  load  current 
and  line  pressure  550  volts,  as  in  the  case  of  motor  A. 
existing  between  current  and  speed  can  be  calculated 
ceding  case,  the  results  being  given  in  Table  XII. 


resistance  are 
-voltage  curve 
amperes  field 
densities  than 
is  50  amperes 
The  relations 
as  in  the  pre- 


TABLE    XII.  — CURRENT-SPEED    DATA,   SERIES   MOTOR    B    (HIGH 

FLUX  DENSITY). 


Amp. 

A 

B 

C 

C.e.m.f.= 
A-(B+C) 

Volts  at  200  r.p.m. 
Curve  B,  Fig.  2. 

200  X  c.e.m.f. 

V. 

«. 

IR» 

Volts  at  200  r.p.m. 

5 

550 

2 

5 

543 

137 

200  (543  -hi  37)  =  794 

10 

550 

4 

10 

536 

235 

200  (536  -=-235)  =  455 

20 

550 

8 

20 

522 

360 

200  (522-  360)  =  290 

30 

550 

12 

30 

508 

420 

200  (508  -5-  420)  =  242 

40 

550 

16 

40 

494 

450 

200  (494^-450)  =  220 

50 

550 

20 

50 

480 

462 

200  (480-^-462)  =  207 

The  effects  of  low  and  high  magnetic  flux  densities  upon  speed  of 
series  motors  at  various  loads  are  shown  by  comparing  the  two  speed- 
current  curves  in  Fig.  53.  For  motor  B  with  high  flux  density  the 
speed  range  is  794  to  207  r.p.m.,  or  3.84  :  i,  while  it  is  1920  to  233 
r.p.m.,  or  8.24  :  i  for  motor  A,  which  is  more  than  twice  as  great  as 
the  variation  in  speed  with  the  motor  of  high  flux  density,  the  current 
change  being  the  same,  that  is,  5  to  50  amperes  in  both  cases.  The 
type  of  direct-current  series  motors  commonly  employed  is  that  with 
the  higher  flux  density  on  account  of  the  greater  economy  of  material 
and  better  operation  under  conditions  of  variation  in  the  line  voltage. 

Comparing  the  speed  curves  of  series  and  shunt  motors,  Fig.  54,  it 
is  apparent  that  the  speed  changes  in  series  motors  are  due  not  only 
to  /  (Ra  +  Rse)  effects  but  more  especially  (up  to  heavier  loads) 
to  increase  in  field  strength.  The  IRa  drop  is  greater  in  series  than 
in  shunt  motors  of  the  same  rating,  because  the  former  are  usually 
designed  for  intermittent  service,  so  that  the  current  density  in  the 
field  and  armature  windings  can  be  made  much  higher  than  would 
be  approved  of  in  shunt  machines,  which  are  usually  loaded  more 
continuously. 


98 


ELECTi 
2000 
1800 
1600 
1400 
g'1200 

4 
MIQOO 

800 
600 
400 
200 

•?/C  MOTORS,    THEIR  ACTION  AND   CONTR( 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\6 

> 

\ 

\ 

^  — 



^^ 

*• 

.  — 

Ml 

—                   — 

- 

~                   — 

„ 
== 

5         10        15        20 


35        40        45       50 


Current 
FIG.  53. SPEED-CURRENT  CURVES  OF  SERIES  MOTORS,  A  AND  B. 


\ 


Sha  lu 


0102030405060708090 
Percent  Rated  Current 


FIG.  54.  —  COMPARATIVE  SPEED  AND  TORQUE  CURVES  OF  IOO-H.P. 
SERIES  AND  SHUNT  MOTORS. 

The  speed  for  any  given  current,  as  already  shown,  depends  upon 
the  c.e.m.f.  generated  or  upon  V  —  I  (Ra  +Rse);  hence  the  speed  at 
any  line  voltage  Vx  is  obtained  from  the  equation 


r.p.m.x  = 


VX-I  (Ra  +  RJ 
V  -  I  (Ra  +  RJ 


r.p.m. 


where  r.p.m.  and  r.p.m.x  are  the  speeds  at  the  standard  and  frac- 
tional  voltages  respectively;  V  and  Vx  being  these  voltages  while 


DIRECT-CURRENT  SERIES  MOTORS. 


99 


/  (Ra  +  Rse)  is  the  resistance  drop  in  the  motor  windings.  When 
Vx  is  less  than  V,  the  speed  corresponding  to  it  is  smaller  than  the 

ratio  —^—    — -  ,  since  the  resistance  drop  is  then  a  larger  part  of  Vx 

than  it  is  of  V.  Similarly  when  Vx  is  greater  than  V  the  reverse  is 
true.  When  Vx  is  equal  to  /  (Ra  +  Rse)  the  armature  will  stand 
still,  exerting  torque  corresponding  to  /.  For  any  increase  in  Vx 
above  this  value  the  speed  rises  proportionately. 


6OOO       6O 


&4O0P   i  40 

\         \ 


0        O 


FIG.  55.  —  SPEED-CURRENT  AND  TRACTIVE  EFFORT-CURRENT  CURVES 
OF  A  SERIES  MOTOR. 

G.  E.  6gC  Railway  Motor.     Gear  Ratio,  1.885;  Wheel  Diam.,  36  ins.; 
Resistance  of  windings,  0.14  ohms. 


By  means  of  the  preceding  speed-current  equations,  the  corre- 
sponding curves  (Fig.  55)  of  the  typical  2oo-horsepower  series  rail- 
way motor  (G.  E.  Type  6gC,  N.Y.C.  "M.  U."  trains)  at  150  and 
300  volts  have  been  calculated  from  the  6oo-volt  curve  given  by  the 
manufacturer. 

Torque-current  Curve.  —  The  torque  of  any  motor  varies  directly 
with  the  product  of  armature  current  and  field  flux.  Hence  in  a  series 
motor  the  torque  at  low  flux  densities  varies  directly  as  the  square 


100       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

of  the  current;  that  is,  torque  =  KP\  but  as  the  magnetization 
approaches  saturation  the  torque  becomes  more  nearly  propor- 
tional to  the  first  power  of  the  current. 

The  torque  of  a  series  motor  is  independent  of  the  voltage  except 
for  variation  in  hysteresis,  eddy-current,  friction  and  windage  losses 
resulting  from  the  change  in  speed  with  the  altered  voltage.  At  low 
voltages  and  corresponding  speeds,  these  losses  are  reduced  and  the 
available  torque  per  ampere  is  similar!^  increased;  at  higher  voltages 
the  reverse  is  true.  Since  the  hysteresis  loss  varies  with  the  first 
power,  and  eddy-current  loss  as  the  square  of  speed,  the  difference  in 
torque  for  any  given  current  is  greater  between  the  150  and  300  volt 
curves  than  that  between  the  300  and  600  volt  curves.  The  per- 
centage difference  between  torque  values  at  any  two  voltages  in- 
creases with  the  current  on  account  of  ratio  of  speeds  at  these 
voltages.  Fig.  55  shows  the  torque-current  curve  for  the  typical 
2oo-horsepower  series  motor  at  600  volts,  being  approximately  cor- 
rect for  150  and  300  volts  also,  because  the  variation  in  the  losses 
is  sTnall  compared  with  the  total  torque. 

The  full-load  value  of  motor  current  in  the  case  of  a  railway  equip- 
ment is  generally  employed  as  the  starting  current. 

A  comparison  of  the  speed-current  and  the  torque-current  curves 
of  a  series  motor  (Fig.  55)  shows  that  the  maximum  torque  exists  at 
the  minimum  speed.  This  is  the  especially  valuable  feature  of  the 
series  motor,  as  maximum  torque  can  thus  be  obtained  at  starting, 
with  consequent  rapid  acceleration.  Further  study  of  these  curves 
also  brings  out  the  bad  feature  of  the  series  motor,  namely,  that  very 
high,  in  fact  dangerously  high,  speeds  may  be  attained  by  the  arma- 
ture if  the  load  be  very  much  reduced.  Series  motors  should,  there- 
fore, be  either  geared  or  directly  connected  to  their  load  to  prevent 
any  breaking  of  the  mechanical  connection  which  is  likely  to  occur 
with  a  belt. 

The  torque  per  ampere  may  be  called  the  torque-efficiency  of  the 
motor.  This  varies  with  the  current,  but  there  is  a  gradual  reduction 
in  its  rate  of  increase,  because  the  magnetic  circuit  approaches 
saturation  as  the  current  becomes  larger.  The  "  torque  per  ampere- 
current"  curve  of  a  series  motor,  Fig.  56,  is  substantially  the  mag- 

T 

netization  curve,  because  T  = 


The  torque  per  ampere  curve  in  Fig.  56  gives  the  torque  in  terms 


DIRECT-CURRENT  SERIES  MOTORS. 


101 


of  pounds  pull  at  i  foot  radius  from  motor  shaft.  This  value  is 
obtained  by  dividing  the  pounds  tractive  effort  per  ampere  by  the 
gear  ratio  and  multiplying  this  quotient  by  the  radius  of  the  wheels 
in  feet.  For  example,  the  torque  per  ampere  at  motor  shaft  with 
armature  current  of  300  amperes  is  3120  X  1.5  -*-  300  X  1.885  = 
8.27  pounds  at  a  foot  radius. 


/o 


8 


0  /OO         ZOO         ZOO         400        5OO 

Asnperes 

FIG.  56.  —  TORQUE  PER  AMPERE-CURRENT  CURVE  OF  A  G.  E.  690 
RAILWAY  MOTOR. 


A  working  value  of  the  constant  K  can  be  obtained  by  determin- 
ing the  torque  T  at  any  reasonable  current  /  and  dividing  this  by 
the  product  of  that  current  and  the  electromotive  force  corresponding 
thereto  when  the  motor  is  operated  as  a  separately  excited  dynamo 
at  any  given  speed. 


102      ELECTRIC  MOTORS,    THEIR   ACTION   AND   CONTROL. 

Horsepower-current  Curves.  —  The  output  is  readily  calculated 
from  the  torque  and  speed,  since  torque  is  expressed  as  pounds 
pull  at  a  one-foot  radius.     The  output  is  derived  as  follows: 
2  x  torque  (r.p.m.)  _  torque  (r.p.m.) 

525o 


h.p. 


360 


320 


280 


2*40 


00 


20 


80 


40 


0  /OO         200         300       400       50O 

A/nperes 

FIG.  57.  —  BRAKE  HORSEPOWER-CURRENT  CURVES  OF  A  G.  E.  690 
RAILWAY  MOTOR. 

If  the  turning  moment  of  the  motor  be  expressed  as  tractive  effort 
or  pounds  pull  at  the  rim  of  the  car  wheel  (t.e.)  and  the  speed  in  miles 
per  hour  (m.p.h.),  then  the  output  is  given  by 

(t.e.)  (m.p.h.) 

h'p'=  '  ~^T  - 


DIRECT-CURRENT  SERIES   MOTORS. 


103 


The  horsepower  output  for  a  given  current  varies  almost  propor- 
tionally with  the  voltage;  thus  at  150  and  300  volts  the  horsepower 
output  is  practically  one-quarter  and  one-half,  respectively,  of  the 
value  at  600  volts.  More  exactly,  the  output  at  sub-voltages  is  a 
little  less  than  the  same  fractional  part  of  the  output  at  600  volts, 
since  the  percentage  increase  of  torque  at  the  reduced  voltage  is  less 
than  the  corresponding  decrease  of  speed.  Fig.  57  shows  the  curves 
of  horsepower  output  and  current  at  600,  300  and  150  volts  for  the 
2oo-h.p.  motor  previously  considered. 


zoo        JOG        400       wo 


FIG.  58:  —  EFFICIENCY-CURRENT  CURVES  OF  A  G.  E.  6pC  RAILWAY  MOTOR 

(GEAR  LOSSES  INCLUDED). 


Efficiency-current  Curves.  —  The  efficiency  curves  of  this  typical 
2oo-h.p.  series  motor  at  150,  300  and  600  volts  in  Fig.  58  show 
that  the  efficiency  of  such  a  motor  at  rated  voltage  is  nearly  constant 
over  a  very  wide  range  of  speed  and  load,  but  with  very  small  or 
excessively  large  currents  it  falls  to  low  values.  At  voltages  less 
than  normal,  the  efficiency  is  reduced,  and  this  reduction  is  more 
marked  as  the  voltage  is  lowered. 


104       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

The  efficiency  at  any  load  may  be  readily  determined  by  any  of 
the  following  formulae: 


Efficiency 


746  (h.p.  output) 
watts  input 

0.142  torque  (r.p.m.) 
watts  input 

1.99  (t.e.)   (m.p.h.) 
watts  input 


(14) 


tooo      mo 
Tractive  Effort  in  Pounds 


FIG.  59.  —  SPEED  TRACTIVE  EFFORT  CURVES  OF  A  G.  E.  6pC  RAILWAY  MOTOR. 

Speed-Tractive  Effort  Curves. — This  is  the  most  important  charac- 
teristic of  a  railway  series  motor  in  the  determination  of  its  fitness 
for  a  specified  service.  The  speed-t.e.  curves  are  similar  in  form  to 
the  speed-current  curves,  due  to  the  fact  that  the  t.e.-current  curve 
is  almost  a  straight  line.  The  equation  for  the  speed-tractive  effort 
curve  is  of  the  form 


t.e. 


m.p.h.  +  B 


C. 


(15) 


DIRECT-CURRENT  SERIES  MOTORS.  105 

The  constants  change  with  the  voltage,  resistance  and  degree  of 
magnetization  of  the  magnetic  circuit.  Fig.  59  gives  the  speed- 
tractive  effort  curves  of  the  typical  2oo-h.p.  series  motor  at  150,  300 
and  600  volts,  the  equations  of  which  are: 


At  150  volts,  t.e. 
i 
At  300  volts,  t.e. 

At   fSnn  volte;     t  p 

12,800 

m.p.h.  —  1.82 
25>3°° 

970. 

m.p.h.  —  5.78 
35>8o° 

-  1050. 

-  >7/in 

7       -  i  ,  /   T 

m.p.h. —  16.43 

The  insert  points  as  shown  in  Fig.  59  are  derived  by  calculation 
and  agree  very  closely  with  the  test  values  over  a  wide  range  of  speed. 

Gears  and  Wheels.  —  A  railway  motor  is  usually  connected  to  the 
axle  of  the  car  drivers  through  a  single  pinion  and  gear,  but  in  some 
cases  the  armature  is  directly  mounted  upon  the  axle  of  the  car 
wheels.  The  speed  and  tractive  effort  at  the  rim  of  the  driving 
wheels  are  respectively  proportional  and  inversely  proportional  to 
the  wheel  diameter,  while  with  gearing  these  two  quantities  are 
dependent  upon  the  gear  ratio,  which  is  always  designed  to  secure 
speed  reduction.  The  proper  ratio  depends  upon  the  type  of  service 
to  be  performed,  but  usually  lies  between  2  and  5,  the  lower  value 
corresponding  to  high-speed  service.  The  effect  on  tractive  effort 
and  speed  of  larger  wheel  diameter  is  exactly  the  reverse  of  that 
obtained  by  increasing  the  gear  ratio. 

The  speed  and  tractive  effort  for  any  gear  ratio  and  wheel  diameter 
may  be  found  for  any  other  known  conditions  from  the  following 
formulae,  wherein  m.p.h.  and  m.p.h. x  are  respectively  the  known  and 
the  unknown  speeds,  in  miles  per  hour,  for  an  existing  gear  ratio  r 
and  wheel  diameter  D.  The  gear  ratio  rx  and  wheel  diameter  Dx 
correspond  to  the  unknown  speed  (m.p.h. x).  T  is  the  torque  for 
the  known  gear  ratio,  and  Tx  is  the  corresponding  unknown  value. 

7~)  f*  ,^  ~       /7«    -p>     Y-n     \       7")  r*.f*  -7       /f    TV     fY\     \       7~) 

m.p.h.x  =  m.p.h. = = •  (16) 

JT  ^  T\  \  ' 


106       ELECTRIC   MOTORS,    THEIR   ACTION   AND   CONTROL. 

The  characteristic  curves  of  a  standard  Westinghouse  railway 
motor  with  gear  ratio  of  22  :  62  are  shown  in  Fig.  60.  This  has  con- 
siderably smaller  power  than  the  typical  General  Electric  machine 
to  which  the  preceding  curves  relate,  the  maximum  current  being 
200  amperes  for  the  former  and  500  for  the  latter. 


WESTINGHOUSE  RAILWAY  MOTOR 
500  VOLTS 

GEAR  RATIO.  22  TO  62-33" WHEELS 
CONTINUOUS 


FIG.  60.  —  CHARACTERISTIC  CURVES  OF  A  TYPICAL  RAILWAY  MOTOR. 

Motor  Losses.  —  The  rated  output  of  a  motor  is  determined  by  its 
commutation  and  heating  limits;  hence  even  a  small  reduction  of  the 
losses  within  the  motor  is  of  considerable  importance.  For  example, 
assume  th2  efficiency  of  a  motor  to  be  increased  from  90  to  91  per 
cent,  a  difference  of  but  i  per  cent,  in  which  case  the  total  loss  within 
the  motor  is  reduced  from  10  to  9  per  cent,  resulting  in  a  decrease  of 
10  per  cent  in  the  heat  to  be  radiated  and  a  probable  lowering  of  the 
temperature  rise  by  nearly  10  per  cent.  Thus  it  is  seen  that  the  rated 
output  of  a  motor  depends  largely  upon  its  efficiency.  The  per- 
missible output  is  not,  however,  increased  10  per  cent  in  the  above 
example,  because  the  heating  effect  is  as  the  square  of  the  current. 
For  example,  when  current  rises  from  i.oo  to  1.05,  the  heating  effect 
is  augmented  from  i.oo  to  1.1025.  On  the  other  hand,  the  PR  heat 


DIRECT-CURRENT  SERIES  MOTORS. 


107 


is  only  about  one-half  of  the  total  (including  core  losses),  so  that 
armature  current  and  output  may  be  raised  say  7  per  cent.  The 
increased  output  due  to  rise  in  field  current  would  not  be  great, 
because  the  flux  usually  approaches  saturation  at  maximum  current 
and  may  be  assumed  to  be  i  or  2  per  cent  additional. 

The  distribution  of  the  losses  between  field  and  armature  is 
of  wide  variation.  The  following  tables  taken  from  a  paper  by 
W.  B.  Potter  give  values  of  the  loss  distribution  of  typical  railway 
motors.* 


TABLE  XIII.  — LOSSES  AT  RATED  LOAD  IN  PER  CENT  OF  OUTPUT. 


Armature. 

Commercial 

Field 

Motor 

Rating,  h.p. 

/2/e. 

Total. 

I2R. 

Core. 

Total. 

38 

4.70 

4.00 

2.37 

6.37 

11.07 

38 

4.60 

3.80 

4.92 

8.72 

13.32 

50 

4.20 

2.10 

3.45 

5.55 

9.75 

50 

3.25 

2.80 

4.80 

7.60 

10.82 

50 

4.33 

3.36 

4.17 

7.53 

11.86 

75 

3.20 

2.50 

2.93 

5.43 

8.63 

125 

2.48 

2.40 

2.12 

4.52 

7.00 

TABLE  XIV.  — SEGREGATED  LOSSES  AT  RATED  LOAD  IN  PER  CENT 

OF  TOTAL  LOSSES. 


Armature. 

Ratio 

Commercial 

Field 

Field  Loss 

Rating,  h.p. 

I*R, 

to 

I*R. 

Core. 

Total. 

Armature  Loss. 

38 

42 

36 

22 

58 

.74 

38 

35 

28 

37 

65 

.53 

50 

43 

22 

35 

57 

.76 

50 

30 

26 

44 

70 

.43 

50 

37 

28 

35 

63 

.57 

75 

37 

29 

34 

63 

.59 

125 

36 

34 

30 

64 

.55 

Rating.  —  There  are  two  ratings  by  which  railway  motors  are 
commercially  classified.  The  nominal  rating  of  the  General  Electric 
Company  is  the  better  known.  This  is  defined  as  that  output  which 

*  Trans.  Amer.  Inst.  Elect.  Eng.,  Vol.  XIX  (1902),  p.  170. 


108       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL 

causes  a  temperature  rise  of  75  degrees  C.  from  a  room  tempera- 
ture of  25  degrees  C.  after  an  hour's  run  upon  test  stand  with  motor 
covers  open  and  500  volts  at  motor  terminals.  This  rating  is  much 
in  excess  of  the  continuous  service  capacity  of  the  motor,  but  it  gives 
a  convenient  means  of  classification  and  is  a  severe  test  upon  the 
mechanical  qualities  of  the  machine.  In  view  of  the  tendency  to 
use  higher  voltages,  a  test  at  550  volts  in  place  of  500  volts  would 
probably  more  nearly  approach  service  conditions  at  the  present 
time. 

The  continuous  rating  used  by  the  Westinghouse  Company  cor- 
responds to  that  current  which  supplied  continuously  to  the  motor 
on  a  test  stand  will  produce  a  temperature  rise  of  60  degrees  C. 
The  voltage  selected  is  an  approximation  of  the  average  value 
throughout  the  period  of  starting  as  well  as  running.  No  attempt 
is  made  to  reproduce  the  conditions  of  ventilation  that  obtain  in 
actual  service. 

The  instantaneous  capacity  of  a  railway  motor  is  limited  by  its 
commutation;  nevertheless  in  well-designed  car  equipments  the 
motor  should  be  able  to  slip  the  wheels  under  normal  track  condi- 
tions before  the  commutation  limit  is  reached. 

The  rating  of  railway  motors  must  necessarily  be  largely  arbi- 
trary because  of  the  intermittent  character  and  very  variable 
conditions  of  such  service.  The  only  conclusive  test  is  actual 
operation  under  the  practical  conditions  of  each  particular  road, 
which  a  shop  test  can  only  approximate  in  a  general  way.  It  is 
well,  however,  to  have  some  nominal  rating,  as  defined  above,  in 
order  to  estimate  and  compare  the  performance  of  different  railway 
motors. 


CHAPTER    X. 

CONTROL   OF   DIRECT-CURRENT    SERIES    MOTOR. 

Function  of  Controller.  —  For  all  series  motors,  with  the  possible 
exception  of  those  operating  small  fans  and  pumps,  a  starting  device 
is  necessary  to  increase  gradually  the  voltage  applied  to  motor 
terminals.  By  this  means  the  starting  current  and  acceleration  are 
regulated,  while  the  heating  and  sparking  of  the  motor  are  restricted 
within  reasonable  limits.  Starting  is  accomplished  by  inserting 
proper  values  of  resistance  in  series  with  the  motor,  usually  supple- 
mented by  change  from  series  to  parallel  connection  of  two  or 
more  motors. 

Rheostat  Control. — The  simplest  form  of  control,  Fig.  51  (page  94), 
is  by  means  of  series  rheostat  or  resistance  (R)  which  is  gradually  re- 
duced in  predetermined  steps  until  the  motor  terminals  are  connected 
directly  across  the  full  line  voltage.  This  method  is  practically  the 
same  as  the  rheostat  control  of  shunt  motors,  except  that  only  the 
armature  current  is  affected  in  the  latter  machine.  The  same 
objections  apply,  however,  in  both  cases.  These  include  bulkiness 
of  rheostat,  widely  varying  speed  with  any  considerable  change  in 
torque  and  very  low  efficiency.  For  example,  the  loss  by  this 
control  with  uniform  acceleration  is  practically  one-half  the  total 
energy  supplied  by  the  line  during  the  period  of  starting  and  thus 
equals  the  energy  consumed  in  the  motor.  Moreover,  the  only 
efficient  running  speed  is  the  full  value. 

Series-parallel  Control.  — Where  two  or  more  motors  or  two  or 
more  windings  on  the  same  armature  are  to  be  controlled  simul- 
taneously, certain  groupings  may  be  obtained  by  means  of  which 
a  single  motor  or  winding  receives  but  a  fraction  of  the  line  voltage 
without  external  resistance  in  circuit.  Thus,  two  motors  operating 
together  may  be  connected  in  series  with  each  other  and  in  series 
with  starting  resistance,  which  is  gradually  reduced  until  each  motor 
receives  one-half  line  voltage.  The  motors  may  then  be  thrown  in 
parallel  with  each  other  and  in  series  with  resistance,  which  is 
again  cut  out  in  steps  until  each  motor  receives  full  voltage.  Thus 

109 


110       ELECTRIC   MOTORS,    THEIR   ACTION   AND   CONTROL. 


there  are  two  points  of  the  control  at  which  no  resistance  is  in  cir- 
cuit. The  total  energy  loss  during  the  period  from  starting  to  full 
parallel  operation  is  approximately  one-third  of  the  line  supply  or 
one-half  of  the  motor  consumption. 

The  speed  of  the  motors  when  operated  in  series  is  about  one-half 
that  with  the  parallel  connection.  This  series-parallel  control  is  the 
method  generally  adopted  for  single  cars  and  for  multiple  unit  trains 


TWO  MOTORS.  FOUR  MOTORS. 

FIGS.  6l  AND  62.  —  CONNECTIONS  OF  SERIES -PARALLEL  CONTROL  FOR 
TWO  AND  FOUR  MOTOR  EQUIPMENTS. 

whenever  two  or  more  motors  are  operated  simultaneously.  The 
various  steps  of  this  method  of  control  are  diagrammatically  illus- 
trated in  Figs.  61  and  62,  which  are  respectively  for  two  and  four 
motor  equipments. 

Series,  Series-parallel,  Parallel  Control.  —  When  the  equipment 
consists  of  four  motors,  it  is  a  common  arrangement,  especially  for 
electric  locomotives,  to  employ  three  groupings  of  motors  in  start- 
ing them,  as  illustrated  in  Fig.  63.  At  first  the  four  motors  are 
all  in  series  (A),  then  two  groups,  each  consisting  of  two  motors 
in  series,  are  connected  in  parallel  (B),  while  finally  the  four 
motors  are  placed  in  multiple  (C).  During  the  first  few  steps  of 
each  combination,  series  resistance  is  gradually  cut  out  of  the 
circuit.  This  method  secures  higher  efficiency  than  the  others  but 


CONTROL  OF  DIRECT-CURRENT  SERIES  MOTOR.  Ill 

necessitates  more  car  wiring  than  either  the  simple  rheostatic  or 
series-parallel  control.  The  energy  loss  with  this  method  of  control 
during  the  period  of  acceleration  is  about  f\  of  the  total  supply,  or 
about  f  of  the  motor  consumption.  The  three  efficient  running 
points  are  as  shown  in  Fig.  63,  and  they  correspond  approximately 
to  one-quarter,  one-half  and  full  speed. 


kD— rows 


FIG.  63.  —  RESISTANCE  CONNECTIONS  OF  THE  THREE  RUNNING  POINTS  OF  THE 
SERIES,  SERIES -PARALLEL,  PARALLEL  METHOD  OF  CONTROL. 

Field  Control. —  During  the  early  development  of  electric  traction 
it  was  customary  to  increase  speeds  after  the  efficient  running  points 
(without  external  resistance)  had  been  reached  by  shunting  some  of 
the  field  winding  or  by  arranging  the  field  coils  in  parallel  com- 
binations (field  commutation).  The  resulting  weakening  of  the 
field,  however,  led  to  objectionable  sparking  at  the  brushes,  and 
this  means  of  control  for  railway  motors  was  discarded.  The  intro- 
duction of  interpoles  or  commutating  poles,  which  maintain  a 
commutating  flux  independent  of  the  condition  of  the  main  field, 
but  proportional  to  the  armature  current,  has  revived  the  use  of 
field  control  for  high-speed  railway  motors. 

The  performance  curves  of  a  35-horsepower  series  motor  con- 
trolled in  this  manner  are  given  in  Fig.  64,  while  Fig.  65  illustrates 
the  various  steps  of  such  a  method  of  speed  regulation.* 

The  control  is  by  series-parallel  connection  supplemented  by  the 
increase  of  speed  obtainable  with  field  weakening.  After  starting 
with  resistance  in  the  ordinary  way,  the  machines  are  brought 
into  the  series  ^running  condition  (No.  5)  with  full-speed  strength. 
Higher  speeds  are  attained  by  combining  the  four  field  coils  of  each 
motor  in  partial  series-parallel  (No.  6),  then  in  series-parallel  (No. 

*  Article  by  G.  H.  Condict,  Electrical  World,  Vol.  XLVII,  1906,  p.  1088. 


112      ELECTRIC   MOTORS,    THEIR  ACTION   AND   CONTROL. 


2000 


1500 


1000 


40          50          60          70          80          90         100 
Amperes 

FIG.    64.  —  CURVES    OF    35'H.P.    E.  D.    CO.    SERIES    MOTOR  WITH  FIELD  CONTROL. 


POSITIONS 
START  RUN 
1 


3 
4 

5 

6 


— rn~n — o-^n» 
— tftrn — o- 
— Efftn — o*w* 

MlTtl— Ostfr- 


10 


11 


•O-    Armature 
Main  Field 
Aux.      " 
Resistance 


FIG.    65.  —  CONNECTIONS   FOR   FIELD   CONTROL    OF    SERIES    MOTORS. 


CONTROL  OF  DIRECT-CURRENT  SERIES  MOTOR.  113 

7)  and  finally  in  parallel  (No.  8),  thus  securing  four  field  strengths 
for  any  given  armature  current.  The  speed  rises  as  the  field 
current  is  weakened  but  not  in  equal  degree.  Similar  combinations 
of  the  field  coils  are  employed  with  the  parallel  grouping  of  the 
motors.  Thus  by  this  method  of  control  eight  efficient  running 
points  are  obtained  with  a  two-motor  equipment. 

The  speed  range  at  rated  load,  or  even  at  any  intermediate  load,  is 
not,  however,  8  to  i,  because  the  field  strength  does  not  change 
directly  with  the  m.m.f.,  the  magnetic  circuit  being  partially  saturated. 

It  is  apparent  from  the  characteristic  curves  of  this  equipment 
(Fig.  64)  that  the  speed  with  armature  and,  field  windings  of  both 
motors  all  in  series,  at  full  field  strength  and  at  rated  load,  is  about 
5.2  miles  per  hour,  while  with  the  two  motors  in  parallel  and  all  field 
coils  in  parallel  (weakest  field  condition  No.  12)  the  speed  is  about 
26  miles  per  hour,  a  range  of  5  to  i.  This  is  a  large  gain  over  the 
ordinary  series-parallel  arrangement,  which  gives  about  2  to  i  range 
of  speed. 

Drum  and  Master  Controller.  —  For  the  smaller  series  motors 
such  as  are  used  for  ordinary  cars,  hoists,  pumps,  fans,  etc.,  the 
power  circuits  are  made  and  broken  in  the  controller  by  means  of 
stationary  fingers  and  movable  contacts  mounted  upon  a  drum  or 
cylinder.  Where  large  currents  are  required,  the  circuits  are  made 
and  interrupted  by  separate  switches,  called  contactors,  which  are 
controlled  electrically  from  a  master  controller  and  operated  by  a 
pneumatic  or  solenoid  device.  This  latter  form  of  control  is  almost 
always  used  where  several  cars  are  to  be  operated  together,  and 
from  any  car  irrespective  of  sequence.  The  control  circuits  are 
made  continuous  from  the  first  to  the  last  coach  by  means  of  a 
train  line  and  "jumpers." 

Hand  and  Automatic  Starting.  —  The  rate  of  acceleration  in 
starting  depends  upon  how  rapidly  the  operator  moves  his  con- 
troller handle  from  the  first  to  the  last  notch.  Some  recent  types 
either  prevent  the  operator  from  passing  to  the  next  point  until  the 
current  has  decreased  to  a  certain  value,  or  cause  the  controller  to 
move  ("notch  up")  automatically  at  the  proper  rate  to  a  point 
predetermined  by  the  operator. 

The  efficiency  of  control  methods  may  be  easily  calculated,  assum- 
ing that  the  current  per  motor  is  maintained  constant  by  a  gradual 
increase  in  voltage  at  the  motor  terminals.  This  is  the  ideal  condi- 


114       ELECTRIC   MOTORS,    THEIR   ACTION   AND   CONTROL. 


m 


mi 


il 


•3     «, 


* 


KIl 


. 

KS. 

I 

a 


CONTROL  OF  DIRECT-CURRENT  SERIES  MOTOR. 


115 


tion  because  it  secures  uniform  acceleration  and  is  closely  approxi- 
mated in  practice  with  automatic  starting.  A  comparison  of  the 
three  methods  of  control  described  above  is  set  forth  in  Figs.  66,  67 
and  68.  In  these  and  the  formulas  accompanying  them  /  is  the  cur- 
rent per  motor,  Rm  is  the  resistance,  E  is  line  voltage  and  T  the  time 
from  start  to  full  speed  (period  of  acceleration) .  It  should  be  noted 
that  the  assumption  made  in  developing  these  diagrams  is  that  the 

time  during  which  the  motors  are  in  series  bears  the  same  relation  to 

,-p  . 

the  total  time  of  starting  as  I IRm  j  bears  to  (E  -  IRm). 

The  shaded  portion  of  the  diagrams  represents  the  losses  during 
the  period  of  starting  with  uniform  acceleration  by  each  of  the  three 
methods. 

The  following  values  are  taken  from  a  case  in  actual  practice  and 
the  table  below  is  calculated  from  them. 

Line  pressure  =  E  =  600  volts. 

Current  per  motor  =  /  =250  amps. 

Motor  resistance,  (Ra  +  Rse)  =  Rm  =0.14  ohm. 

Time  occupied  in  starting  =  T  =  30  sec. 

Number  of  motors  =  4. 

COMPARISON  OF  DIFFERENT  METHODS  FOR  STARTING  SERIES 
MOTORS  WITH  UNIFORM  ACCELERATION. 


Items. 

Rheostatic 
Control. 

Series-parallel 
Control. 

Series, 
Series-parallel, 
Parallel  Con- 
trol. 

Supply  for  4  Motors,  kw.-sec  

18000 

13800 

12900 

Motor  Consumption,  kw.-sec  

9500 

9500 

9500 

Total  Controller  Loss,  kw.-sec  
Relative  Controller  Losses  
Relative  Input  to  whole  Equipment.  . 
Total  Efficiency  .  . 

8480 
1.00 
1.00 
0  53 

4240 
0.50 
0.77 
0  69 

3340 
0.39 
0.72 
0  74 

An  examination  of  the  results  set  forth  in  the  above  table  brings 
out  the  great  advantages  of  the  series,  series-parallel,  parallel  method 
of  control,  not  only  as  regards  the  greater  speed  range,  but  also  the 
higher  efficiency  of  the  equipment  during  the  period  of  starting. 

Constant-current  Series  Motors.  — Although  not  used  commer- 
cially at  present,  this  type  of  machine  is  of  such  historical 
importance,  and  differs  so  radically  from  the  constant-potential 


116       ELECTRIC  MOTORS,    THEIR   ACTION   AND   CONTROL. 


Strain  InsuUUr 


motors  now  universally  adopted,  that  it  deserves  some  atten- 
tion. Electric  motors  were  first  used  in  considerable  numbers 
about  1886  or  1887.  Then,  and  for  some  time  afterward,  there 
were  constant-current  arc  lighting  circuits  in  many  smaller  towns 
and  in  portions  of  large  towns  where  constant-potential  supply 
was  not  available.  In  such  cases  motors  were  operated  on  these 
circuits  in  series  with  arc  lamps.  The  connections  are  represented 

in  Fig.  69,  A  being  the  armature  and 
F  the  field  circuit  of  such  a  motor, 
the  windings  of  which  were  designed 
to  carry  the  constant  current  (usually 
10  amperes)  continuously.  Hence,  no 
starting  box  was  required,  the  motor 
being  introduced  directly  into  the 
circuit  by  the  cut-out  switch  5. 

A  plain  series  motor  with  constant 
current  exerts  constant  torque  (at  full 
value),  because  T  =  K3>I°  in  which 
field  flux  4>  and  armature  current  /  are 
both  constant.  Unless  the  load  hap- 
pens to  equal  this  torque  some  means 
for  adjusting  the  motor  torque  to  the 
load  must  be  provided,  otherwise  the 

The 


OF 


motor  will  either  stop  or  race. 

SERIES  * 

usual    method    was    to    short-circuit 


FIG.      69.    CONNECTIONS 

CONSTANT-CURRENT      SI 
MOTOR. 

more  or  less  of  the  field  winding,  or  to 

shunt  it  with  variable  resistance  by  means  of  a  switch  controlled  by  a 
centrifugal  governor  mounted  on  the  motor  shaft.  A  diminution 
in  load  below  the  motor  torque  would  produce  a  rise  in  speed,  and 
the  governor  would  then  reduce  the  field  strength  and  torque  to  the 
proper  value.  The  speed  of  these  constant  current  motors  was 
also  regulated  by  shifting  the  brushes  or  by  moving  the  arma- 
ture out  of  the  field.  The  serious  objection  to  the  operation  of 
motors  on  constant-current  series  circuits  is  the  high  e.m.f.  of  the 
latter,  usually  from  3000  to  6000  volts.  Even  the  potential  differ- 
ence between  the  motor  terminals  is  about  100  volts  (1000  watts  -5- 
10  amperes)  per  horsepower,  which  would  amount  to  1000  volts  for  a 
lo-horsepower  machine.  All  of  this  except  30  or  40  volts  drop  in 
the  field  winding  (IRse)  would  exist  as  a  potential  difference  between 


CONTROL  OF  DIRECT-CURRENT  SERIES  MOTOR.  117 

the  brushes,  being  dangerous  to  persons  and  high  for  commutation. 
To  supply  50  horsepower  to  a  factory  by  this  system  would  involve 
a  potential  difference  of  5000  volts  between  the  two  conductors, 
whether  using  one  motor  or  several  in  series.  The  constant-current 
motor,  however,  possesses  the  great  advantage  that  the  armature 
may  be  stopped  indefinitely  with  full  current  flowing  and  no  injury 
results.  In  fact,  its  temperature  is  practically  the  same  as  at  rated 
speed,  because  the  absence  of  armature-core  losses  when  standing 
still  usually -makes  up  for  decreased  ventilation. 

For  further  information  concerning  series  motors  see  the  following  : 

AMERICAN  ELECTRIC  RAILWAY  PRACTICE.     Herrick  and  Boynton.     1907. 

DIE  GLEICHSTROMMASCHINE.     Vo.l  II,  p.  630.     E.  Arnold.     1904. 

ELECTRIC  MOTORS  FOR  RAILWAY  SERVICE.  W.  B.  Potter,  Trans.  A.  I.  E.  E., 
Vol.  XIX,  p.  170. 

ELECTRICAL  TRACTION,  Vol.  I.     Wilson  and  Lydall.     1907. 

ELECTRIC  JOURNAL,  Vol.  I,  p.  479;  Vol.  Ill,  pp.  14,  525;  Vol.  IV,  p.  454. 

ELECTRIC  RAILWAY  ENGINEERING.     Parshall  and  Hobart.     1907. 

ELECTRIC  RAILWAYS.    McGraw  Co.     1907. 

TRANS.  A.  I.  E.  E.,  Vol.  XXIV,  p.  65;  Vol.  XXVI,  p.  1407. 

STANDARD  ELECTRICAL  HANDBOOK,  pp.  456,  828,  841.     McGraw.    1908. 

ELECTRICAL  ENGS.'  POCKET  BOOK,  H.  A.  Foster,  pp.  614,  753,  760.  D.  Van 
Nostrand  Co.  1908. 


CHAPTER    XI. 

COMPOUND-WOUND   MOTORS. 

THERE  are  two  classes  of  compound-wound  motors.  In  one  case 
the  ampere-turns  of  the  series  coil  reinforce  the  shunt-field  ampere- 
turns,  producing  the  cumulative-compound  motor;  in  the  second  case 
the  series  ampere-turns  oppose  those  of  the  shunt  winding,  produc- 
ing the  differential-compound  motor.  The  former  machine  is  com- 
mercially called  the  compound  motor,  the  latter  being  known  as  the 
differential  motor.  The  connections  of  a  compound-wound  motor 
are  shown  in  Fig.  70. 


FIG.    70. CONNECTIONS    OF    COMPOUND-WOUND    MOTOR 

As  already  noted,  an  ordinary  shunt  motor  is  usually  operated  with 
constant  potential  at  the  field-circuit  terminals,  so  that  the  flux  is 
practically  constant  at  all  loads  (except  for  a  slight  effect  of  armature 
reaction  which  reduces  this  flux  by  a  small  percentage  at  rated  arma- 
ture current  and  torque).  Hence  the  torque  increases  practically  in 
direct  proportion  to  the  armature  current.  In  a  cumulative-com- 
pound motor  the  conditions  are  somewhat  different,  since  the  field 
becomes  stronger  with  increase  in  load,  due  to  the  magnetizing  action 
of  the  series  coil,  so  that  the  torque  increases  more  rapidly  than  the 
armature  current.  That  is,  the  torque  is  proportional  to  the  arma- 
ture current  and  to  the  field  flux  (due  to  the  sum  of  the  series  and 

V 
shunt  excitations) ,  that  is,  torque  =  KIa  (F h  FIa).    At  the  same 


R 


sh 


118 


COMPOUND-WOUND   MOTORS.  119 

time,  since  the  field  flux  rises  with  increase  in  load,  the  speed 
decreases,  due  not  only  to  IaRa  effects  but  to  the  relation  r.p.m.  = 

.    This  decrease  in  speed  becomes  relatively  less  as  the  load 

n$2p 

increases,  because  the  field  flux  rises  less  rapidly  as  the  magnetic 
density  approaches  saturation,  further  reduction  in  speed  being 
caused  solely  by  the  armature  and  series  field  IR  drop.  Hence  the 
compound  motor  combines  the  characteristics  of  the  shunt  and  the 
series  types,  having  a  speed  not  extremely  variable  under  load 
changes  but  developing  a  powerful  starting  torque.  The  stronger 
the  shunt-field  flux  (no-load  flux)  the  more  nearly  the  action  corre- 
sponds to  that  of  the  shunt  motor;  the  weaker  the  shunt  field  the 
more  closely  does  the  machine  resemble  the  series  motor.  In  fact, 
two  forms  of  cumulative-compound  wound  motors  are  manufac- 
tured, one  with  a  low  ratio  of  series  field  m.m.f.  to  shunt  field  m.m.f. 
varying  from  10  to  25  per  cent  at  rated  load;  the  other  having  a  series 
m.m.f.  equal  to  50  or  75  per  cent  of  the  shunt  field  m.m.f.  at  rated 
load. 

The  characteristic  curves  of  a  4o-horsepower  2  20- volt  compound- 
wound  motor  having  a  field  m.m.f.  at  rated  load  made  up  of  80 
per  cent  shunt  and  20  per  cent  series  excitation,  are  shown  in  Fig.  71. 
The  speed  increase  of  this  machine  between  rated  load  and  running 
free  is  about  35  per  cent.  It  is  interesting  to  compare  the  curves  of 
Fig.  71  with  those  given  in  Fig.  72  as  the  effect  of  weakening  the 
shunt  excitation  and  strengthening  the  series  field  becomes  at  once 
apparent.  The  curves  in  Fig.  72  are  for  a  motor  similar  to  that  of 
Fig.  71,  the  ratio  of  excitations  being  now,  however,  70  per  cent 
series  and  30  per  cent  shunt,  just  enough  of  the  latter  to  prevent  the 
motor  from  racing  when  load  is  entirely  removed;  nevertheless  the 
motor  speed  varies  from  590  to  1010  r.p.m.  The  former  machines 
are  employed  extensively  in  shop  practice  where  a  motor  may  be  re- 
quired to  start  under  heavy  load  but  must  maintain  an  approxi- 
mately constant  speed  after  starting,  or  when  the  load  is  removed. 
The  heavily  compounded  motor  is  employed  where  powerful  starting 
torque  and  resulting  rapid  acceleration  are  necessary  with  a  speed 
not  too  widely  variable  under  load  changes  as  per  the  require- 
ments of  elevator,  rolling-mill  and  similar  service.  In  addition 
to  the  comparatively  constant  speed  required  after  the  running  con- 
dition has  been  attained,  elevator  service  has  an  additional  require- 


120       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 


ment,  namely,  the  motor  must  have  the  series  field-winding  cut  out 
or  short-circuited  at  full  speed  to  avoid  reversal  of  the  machine  should 
the  elevator  be  heavily  overbalanced.  This  would  cause  the  motor 
to  speed  up,  and  if  the  series  winding  is  relatively  powerful  com- 
pared with  the  shunt  winding,  it  would  reverse  the  excitation  and 
result  in  a  burn-out.  Well-designed  shunt  motors  have  a  speed 


H.P 

90 


CO 


50 


20 


10 


B.HP 


•que 


700 


GOO 


500 


a 

400    § 

I 


100 


50 


75  100 

Amperes 


125 


150 


PIG.  71.  —  CHARACTERISTIC  CURVES  OF  A  2O  PER  CENT  4Q-H.P.  COMPOUND 
WOUND  MOTOR. 

regulation  within  5  per  cent;  series  motors  have  a  speed  change  of 
almost  unlimited  ratio  from  no  load  to  full  load  or  vice  versa,  while 
compound-wound  motors  have  speed  variation  from  12  to  100  per 
cent  above  that  corresponding  to  the  rated  torque,  depending  upon 
the  ratio  of  shunt  to  series  field  m.m.f.  as  set  forth  above. 

The  speed  control  employed  with   compound    motors  may  be 


COMPOUND-WOUND   MOTORS. 


121 


any  of  the  various  methods  explained  in  connection  with  the  shunt 
motor,  though  when  used  for  elevator  service  the  control  is  generally 
entirely  rheostatic,  with  the  final  cutting  out  of  the  series  winding 
after  acceleration  has  ceased. 


I.P. 


\ 


I     60 


\ 


B.H.P. 


R.P.M. 


que 


B.P.M. 
900 


500 


o 

"I 


500 


200 


75  100 

Amperes 


150 


200 


FIG.  72.  —  CHARACTERISTIC  CURVES  OF  AN  80  PER  CENT  4O-H.P.  COMPOUND 

WOUND  MOTOR. 

The  Differential  Motor.  — In  this  class  ol  compound-wound 
motor  the  m.m.f  of  the  series  winding  opposes  that  of  the  shunt 
winding  and  thus  weakens  the  field  with  increase  of  load.  The 
object  is  to  compensate  for  IaRa  drop  and  thus  maintain  the  speed 
constant  at  all  loads.  These  machines  will  operate  satisfactorily 
if  not  ovei loaded,  but  if  an  overload  occurs  the  field  flux  becomes  so 
much  reduced  that  the  torque  is  not  sufficient  to  maintain  rotation, 


122      ELECTRIC  MOTORS,   THEIR  ACTION  AND  CONTROL. 

hence  the  c.e.m.f.  falls  to  zero  and  a  burn-out  of  the  armature 
winding  results,  unless  the  machine  is  protected  by  fuses  or  a  circuit- 
breaker.  A  still  further  objection  to  this  motor  is  the  impossibility 
of  obtaining  a  powerful  starting  torque  on  account  of  the  weakening 
of  the  field  with  heavy  currents.  In  fact,  since  the  series  circuit  is 
of  considerably  lower  inductance  than  the  shunt  circuit,  the  motor 
if  quickly  started,  under  load,  would  tend  to  rotate  in  the  wrong 
direction.  Hence  the  series  winding  should  be  automatically  short- 
circuited  during  the  starting  of  the  machine. 

The  same  regulation  as  that  secured  by  differential  winding  may 
be  obtained  from  a  specially  designed  shunt  motor  with  armature 
reaction  exaggerated,  also  by  brush  lag,  by  an  excessive  number  of 
armature  turns,  or  by  proper  use  of  interpoles,  without  the  serious 
weakness  of  the  differential  motor.  For  most  practical  purposes, 
however,  the  speed  of  a  well-designed  shunt  motor  does  not  vary 
to  an  extent  that  is  objectionable,  so  that  this  special  regulation  is 
not  needed.  The  differential  machine  is,  therefore,  rarely  used. 

Resume  of  Characteristics  of  Direct-current  Motors. — 

Shunt  motors  have  a  speed  varying  only  slightly  with  load  changes, 
and  good  starting  torque.  They  are  employed  where  the  speed 
should  be  approximately  constant  and  also  where  the  speed  may  be 
adjusted  by  some  of  the  methods  described. 

Series  Motors.  — These  machines  are  employed  when  powerful 
starting  torque  and  rapid  acceleration  are  demanded,  also  when 
speed  must  be  automatically  adjusted  to  load,  but  they  have  no 
customary  or  even  limit  of  speed  with  variable  loads. 

Compound-wound  motors  have  a  speed  decreasing  considerably 
(12  to  50  per  cent)  with  load,  but  possess  the  powerful  starting 
torque  characteristic  of  series  motors,  and  are  employed  in  connec- 
tion with  work  requiring  that  capability  and  a  speed  not  exces- 
sively variable  under  load  changes.  They  may  be  safely  belted  to 
the  tool,  while  series  motors  should  never  be,  but  must  be  positively 
connected. 

Differential  motors  may  be  designed  to  give  an  almost  absolutely 
constant  speed  under  all  load  changes  within  their  rating,  but 
beyond  this  the  motor  is  too  likely  to  be  stalled;  the  starting  torque 
is  also  small.  This  motor  is  no  longer  used  in  practice,  improve- 
ments in  the  design  of  shunt  motors  having  secured  nearly  constant 
speed  without  the  objectionable  features  of  the  differential  type 


CHAPTER   XII. 

ALTERNATING-CURRENT   MOTORS. 

CLASSIFICATION   AND   HISTORY. 

THE  salient  facts  in  the  historical  development  of  the  electric 
motor  were  briefly  stated  in  Chapter  I.  These  related  chiefly  to 
direct-current  types  because  the  only  source  of  electrical  energy 
commercially  available  up  to  1880  or  thereabouts  was  the  voltaic 
battery.  Furthermore,  the  electrical  generating  plants  employing 
dynamo-electric  machinery  in  operation  prior  to  1890  were  almost 
all  designed  to  supply  direct  currents  only.  Hence  the  commercial 
progress  and  as  a  natural  result  the  scientific  advance  of  the  a.  c. 
motor  were  held  back,  while  the  d.  c.  machine  received  much 
more  attention.  Nevertheless,  during  all  this  time  experiments 
were  being  made  and  ideas  evolved  which  led  up  to  the  various  a.  c. 
types  now  known,  these  being  quite  numerous  and  differing  widely 
from  one  another.  On  the  other  hand,  d.  c.  motors  are  practically 
all  of  one  species,  which  may  be  defined  as  embodying  essentially 
a  drum  armature  with  commutator  in  a  bipolar  or  multipolar  field, 
and  is  the  same  as  the  d.  c.  generator  except  for  mere  differences 
in  form  to  suit  particular  conditions,  as,  for  example,  those  of 
electric  railway  service.  While  there  is  only  one  important  kind  of 
d.  c.  machine,  we  have  the  following  distinct  Types  of  Alternating- 
current  Motors. 

1.  Synchronous  Motor,  being  an  ordinary  a.c.  generator  reversed 
in  function. 

2.  Induction  Motor,  having  armature  winding  closed  upon  itself 
or  through  a  local  circuit  not  connected  to  source  of  supply  and 
without  commutator. 

3.  Repulsion   Motor,    having   armature   with   commutator   and 
brushes  connected  through  local  circuit  wherein  current  is  induced. 

4.  Similar  to  d.  c.  motor,  having  armature  (with  commutator) 
and  field  winding  both  connected  to  supply  circuit,  hence  often 

123 


124       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

called  conductive  motor  in  contradistinction  to  the  two  foregoing 
types. 

5.  Shaded  Pole  or  Creeping  Field  Motor,  in  which  the  phase  of 
the  flux  is  retarded  in  parts  of  the  field. 

In  addition  to  the  above  classification  a  further  differentiation 
of  a.  c.  motors  is  based  upon  the  fact  that  either  of  the  first  two 
types  may  be  operated  by  single,  two,  or  three  phase  currents.  The 
other  three  types  are  supplied  with  single-phase  current,  and  even 
when  fed  from  polyphase  circuits  each  motor  is  connected  to  one 
phase  only. 

It  is  to  be  noted  that  the  conductive  type,  No.  4  of  the  above 
classification,  is  the  only  one  capable  of  being  operated  either  as  a 
d.  c.  or  as  an  a.  c.  motor.  With  the  entire  field  as  well  as  armature 
iron  laminated  and  with  certain  features  to  limit  sparking,  as 
explained  later,  the  same  machine  will  run  well  with  either  direct 
or  alternating  current;  in  fact  many  electric  locomotives  are  so 
operated  on  the  New  York,  New  Haven  and  Hartford  Railroad. 
The  other  forms,  types  Nos.  i,  2,  3,  and  5  of  the  foregoing  list,  are 
incapable  of  operating  with  d.  c.  supply.  It  is  true  that  type  3,  the 
repulsion  motor,  has  a  commutator  and  is  structurally  similar  to 
the  d.  c.  machine;  nevertheless  it  must  be  differently  connected 
and  current  supplied  to  its  armature  by  conduction  instead  of  in- 
duction, in  order  that  it  may  run  as  a  d.  c.  motor.  Hence  it  no 
longer  belongs  to  the  repulsion  class. 

It  is  evident  from  these  statements  that  we  have  only  a  single 
important  type  of  d.  c.  motor  or  generator,  while  there  are  five 
types  of  a.  c.  motor,  one  of  which  is  the  same  as  the  d.  c.  machine. 
The  only  other  kind  of  dynamo  that  has  been  used  for  d.  c.  genera- 
tion is  the  unipolar  or  homopolar  machine.  This  can  also  be 
operated  as  a  d.  c.  motor  and  as  an  a.  c.  motor.  Therefore  the 
latter  seems  to  include  all  cases  of  the  former,  in  addition  to  which 
it  has  at  least  four  distinct  forms  of  its  own. 

The  complete  history  of  a.  c.  motors  would  include,  therefore, 
an  account  of  the  development  of  each  of  these  five  types,  which 
for  the  most  part  owe  their  existence  to  different  times  and  dif- 
ferent inventors  It  is  sufficient  at  this  point  to  indicate  briefly 
the  principal  historical  facts,  to  be  supplemented  later  by  the  dis- 
cussion of  the  individual  types,  which  contain  references  to  invent- 
ors, authors,  patents,  and  articles. 


ALTERNATING-CURRENT  MOTORS.  125 

The  history  of  the  synchronous  motor  is  essentially  similar  to 
that  of  the  a.  c.  generator  because  the  two  machines  are  structur- 
ally identical;  in  fact  the  same  machine  may  be  used  equally  well 
for  either  purpose.  This  reversibility  was  not  fully  appreciated 
and  applied  by  those  who  first  brought  out  and  experimented  with 
electric  generators  and  motors,  so  that  they  were  designed  quite 
differently  and  their  development  was  to  a  large  extent  independ- 
ently carried  forward.  Nevertheless,  any  improvement  in  the 
generator  was  equally  applicable  to  the  motor,  and  vice  versa.  It 
is  also  a  fact,  as  stated  in  Chapter  I,  that  the  reversibility  of  the 
electric  generator  began  to  be  understood  many  years  ago.  For 
example,  Pacinotti  in  1860  invented  his  ring  armature  for  use  in 
motors  as  well  as  generators  and  later  others  described  machines 
to  perform  both  functions.*  In  1868  Wilde  while  operating  alter- 
nators in  parallel  observed  the  fact  that  the  armature  of  one  of 
them  was  caused  to  oscillate  as  a  motor  when  fed  with  current 
generated  from  another.f  Hopkinson  in  1883  published  the 
theory  of  this  phenomenon  and  showed  that  continuous  rotation 
of  the  motor  could  be  maintained.^  In  conjunction  with  Prof. 
W.  G.  Adams  he  soon  verified  these  conclusions  experimentally 
with  three  De  Meritens  alternators  of  several  horsepower  each,  at 
the  South  Foreland  lighthouse,  the  results  being  given  in  a  paper 
by  Adams  on  "The  Alternate  Current  Machine  as  a  Motor."§ 

The  polyphase  generator  or  motor  may  be  regarded  as  a  com- 
bination of  two  single-phase  machines.  At  the  same  time  the  poly- 
phase synchronous  motor  has"  the  practical  advantage  that  it  is  self- 
starting,  while  the  corresponding  single-phase  motor  requires  some 
auxiliary  means  to  bring  it  up  to  synchronous  speed.  This  advan- 
tage is  a  fortunate  incident,  however,  because  polyphase  systems  owe 
their  great  importance  not  to  this  fact  but  to  their  capabilities  of 
operating  induction  motors  and  their  economy  of  material  in  trans- 
mission lines  as  well  as  in  generators,  etc.  Early  in  1887,  Charles  S. 
Bradley  invented  a  machine  to  operate  either  as  two-phase  generator, 

*  Siemens,  Brit.  Patent  No.  3134  of  1878. 

Deprez,  Brit.  Patent  No.  4128  of  1887. 

Dredges,  Electric  Illumination,  London,  1882,  Vol.  I,  p.  69. 
f  Philosophical  Magazine,  January,  1869. 
j  Lond.  Inst.  Ci"'.  Eng.,  1883. 
§  Soc.  Eng.  and  Elec.,  November  13,  1884. 


126       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

motor,  or  converter,  but  this  invention  is  considered  more  fully 
under  the  following  heading. 

The  evolution  of  the  induction  motor  is  directly  related  to  that  of 
polyphase  currents  and  the  production  of  rotary  magnetic  fields  by 
means  of  such  currents.  Many  pieces  of  apparatus  have  been 
devised  and  physical  facts  noted  which  have  contributed  to  the 
advance  in  this  direction.  Arago  *  in  1824  observed  the  retarding 
effect  upon  the  swinging  of  a  compass-needle  produced  by  sur- 
rounding it  with  a  copper  ring.  He  also  deflected  a  stationary  mag- 
netized needle  by  motion  of  a  copper  disk  immediately  below  it,  and 
with  more  rapid  motion  he  caused  the  needle  to  rotate  continuously 
but  at  a  lower  speed  than  that  of  the  disk.  These  phenomena  are 
basic  in  relation  to  the  induction  motor,  being  due  to  the  force  set  up 
between  a  magnet  and  a  conducting  body  when  they  are  moved  with 
respect  to  one  another  so  that  electric  currents  are  induced  in  the 
latter  by  cutting  the  magnetic  lines  of  the  former.  It  was  not, 
however,  until  Faraday's  discovery  of  magneto-electric  induction  in 
1831  that  the  true  explanation  of  these  phenomena  was  forthcoming. 

Walter  Bailey  read  a  paper  before  the  Physical  Society  of  London 
on  June  28,  1879,  entitled  "A  Mode  of  Producing  Arago's  Rota- 
tions," and  exhibited  a  model  in  which  a  copper  disk  was  caused  to 
rotate  by  progressive  shifting  of  magnetism  among  four  fixed  elec- 
tromagnets, by  throwing  on  and  off  as  well  as  reversing  through 
a  revolving  commutator  the  current  obtained  from  two  primary 
batteries.  Thus  the  Arago  effect  was  produced  without  bodily 
moving  the  magnet,  or  in  other  words,  it  was  what  is  now  known  as 
the  rotary  field.  In  1880  Marcel  Deprez  presented  a  paper  before 
the  Societ^  Francaise  de  Physique  describing  a  motor  which  operated 
by  two-phase  currents.  It  was,  however,  of  the  synchronous  type, 
and  is  only  interesting  as  a  step  of  progress  in  this  direction,  because 
in  1883  he  announced!  an  important  theorem  on  the  production  of  a 
rotary  field  by  the  combination  of  two  alternating  magnetic  fields 
differing  in  phase  by  one  quarter  of  a  period.  Deprez  was  the  first 
to  appreciate  that  this  phenomenon  is  analogous  to  the  mechanical 
production  of  rotary  motion  by  the  combination  of  two  forces  (or 
cranks)  acting  at  right  angles,  and  he  was  also  the  first  to  work  out 
the  theory  of  the  magnetic  case. 

*  Annales  de  Chimie  et  Physique,  XXVII,  363;  XXVIII,  325;  XXXII,  213. 
f  Comptes  Rendus,  Vol.  II,  p.  1193,  1883. 


ALTERNATING-CURRENT  MOTORS.  127 

A  number  of  inventors  had  prior  to  that  time  constructed  or 
published  descriptions  of  generators  for  producing  polyphase  cur- 
rents; for  example,  Wheatstone,  Gramme,*  Cabanellas,f  and  others. 

The  next  important  contribution  was  that  of  Professor  Galileo 
Ferraris,  who  in  1885  built  a  two-phase  motor  having  four  poles, 
two  of  which  were  excited  by  one  alternating  current  and  the  other 
two  by  another  alternating  current  differing  in  phase,  the  rotary 
field  thus  produced  causing  the  armature  to  revolve,  without  any 
electrical  connection  to  the  latter.  This  motor  was  not  exhibited 
till  1888,  on  March  i8th  of  which  year  Ferraris  also  read  a  paper 
before  the  Turin  Academy  in  which  he  set  forth  the  geometric  theory 
of  the  rotary  field  and  described  experiments  illustrating  the  same. 
He  pointed  out  the  fact  that  a  motor  armature  in  which  the  current 
is  generated  by  induction  must  necessarily  rotate  less  rapidly  than 
the  field,  or  in  other  words,  there  must  be  a  slip.  He  employed 
armatures  of  iron,  copper,  as  well  as  of  mercury,  and  suggested  that 
a.  c.  measuring  instruments  could  be  made  in  accordance  with  this 
principle.  He  also  explained  how  two-phase  currents  could  be 
obtained  by  dividing  an  a.  c.  circuit  into  two  branches,  one  induc- 
tive and  the  other  non-inductive,  now  known  as  the  split-phase 
connection. 

Charles  S.  Bradley  on  May  8,  1887,  filed  an  application  for 
U.S.  patent  ^0.390,439)  in  which  he  clearly  showed  and  described 
a  generator  having  a  Gramme  armature  tapped  at  four  equidistant 
points  by  connections  to  four  collector  rings.  This  machine  gener- 
ated two-phase  currents,  one  of  the  objects  stated  being  to  obtain 
larger  output  by  reason  of  the  fact  that  one  current  is  at  a  maximum 
when  the  other  is  zero  and  vice  versa.  This  is  one  of  the  great 
advantages  of  polyphase  apparatus  of  all  kinds,  which  are  usually 
capable  of  giving  more  power  than  single-phase  apparatus  of  equal 
weight.  Bradley  also  stated  that  his  machine  could  be  used  as  a 
motor  if  supplied  with  two-phase  currents  and  that  it  would  give 
out  direct  currents  if  fed  with  alternating  currents,  or  conversely; 
that  is  to  say,  it  was  what  is  now  called  a  rotary  converter.  In 
fact  this  was  the  basic  and  controlling  patent  on  that  machine  in  the 
United  States.  Bradley  in  another  U.  S.  patent  (No.  409,450) 
issued  August  20,  1889,  describes  a  similar  machine  with  three 

*  British  Patent  No.  953  of  1878. 
t  British  Patent  No.  200  of  1881. 


128       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

armature  connections  to  generate  three-phase  currents  or  to  operate 
as  a  motor  when  fed  with  such  currents,  which  is  nothing  less  than  a 
three-phase  system  of  power  transmission,  including  also  the  rotary 
converter  to  supply  d.  c.  railways,  arc  lamps,  storage  batteries,  and 
other  electrolytic  apparatus. 

Nikola  Tesla  in  October,  November,  and  December,  1887,  filed 
applications  for  U.  S.  patents  which  were  issued  in  May,  1888,  as  Nos. 
381,968,  381,969,  and  382,279,  setting  forth  a  generator  to  produce 
two-phase  currents,  connected  to  motor  in  which  a  rotary  field  was 
developed  thereby  and  acted  upon  an  armature  of  iron  to  cause  it  to 
revolve.  It  is  interesting  to  consider  wherein  Tesla's  work  differed 
from  the  early  contributions  which  have  just  been  pointed  out. 
Arago's  disk  and  similar  apparatus  prior  to  that  of  Bailey  were  in  no 
sense  electric  motors,  because  the  motion  of  the  disk  was  merely  the 
result  of  bodily  rotating  the  magnet  by  hand  or  by  a  belt.  The 
device  of  Bailey  was  an  induction  motor,  because  currents  supplied 
to  its  stationary  magnets  set  up  a  rotary  field  which  induced  currents 
in  and  caused  the  revolution  of  its  armature.  On  the  other  hand,  the 
currents  were  not  truly  alternating,  being  merely  currents  from  two 
batteries  which  were  reversed  by  a  commutator  turned  by  hand. 
Furthermore,  what  Bailey  exhibited  in  1879  was  only  a  small  model 
with  a  disk  about  2f  inches  in  diameter,  incapable  of  exerting  any 
appreciable  power,  its  design  being  wholly  unadapted  to  practical 
use.  On  the  contrary,  Tesla  particularly  describes  the  use  of  a  true 
"  alternating  current,  each  impulse  of  which  involves  a  rise  and  fall 
of  potential"  in  order  that  "the  progression  of  the  poles  will  be  con- 
tinuous and  not  intermittent"  as  in  the  case  of  reversed  currents. 
He  also  points  out  "the  practical  difficulty  of  interrupting  or  revers- 
ing a  current  of  any  considerable  strength."  The  drawings  and 
specifications  of  these  Tesla  patents  set  forth  machines  which  are 
evidently  -intended  to  be  used  as  practical  motors. 

The  theory  of  the  rotary  field,  published  by  Deprez  in  1883, 
gave  a  definite  mathematical  basis  for  this  important  physical 
principle,  but  he  did  not  embody  it  in  a  concrete  motor,  and  could 
not  therefore  have  obtained  a  patent  for  his  results,  original  though 
they  were,  since  a  principle  unapplied  is  not  patentable. 

On  the  contrary,  Ferraris  did  work  out  not  only  the  theory 
involved  but  also  constructed  motors  in  accordance  therewith.  In 
this  country,  however,  he  labored  under  the  legal  disadvantage 


ALTERNATING-CURRENT  MOTORS.  129 

that  he  could  get  no  benefit  for  what  he  did  prior  to  his  printed 
publications,  while  Tesla  could  go  back  to  his  earliest  notes,  experi- 
mental work,  and  private  disclosures  to  others.  This  is  the  one 
respect  in  which  a  foreigner's  rights  are  not  equal  to  those  of  an 
American  citizen  in  the  eyes  of  the  patent  law.  It  is  also  true  that 
Ferraris  did  not  appreciate  the  great  practical  value  of  his  inven- 
tion, but  this  is  often  the  case  even  with  the  best  ideas  until  they 
are  applied  and  a  demand  created.  In  itself  this  would  not  invali- 
date his  patent  rights,  especially  as  we  have  seen  that  he  actually 
built  working  induction  motors  operated  by  polyphase  currents 
and  suggested  the  applicability  of  the  same  principle  to  a.  c.  meas- 
uring instruments.  It  would  seem,  therefore,  that  Ferraris  had 
good  moral  claims  to  the  credit  of  the  invention,  but  in  this  country 
was  disqualified  legally  from  using  the  earlier  evidence  in  his 
favor. 

Bradley  in  his  U.  S.  patent  No.  390,439,  already  cited,  which 
was  applied  for  May  7,  1887,  did  not  set  forth  or  claim  the  induc- 
tion motor,  but  he  clearly  showed  and  described  a  machine  to 
serve  as  a  generator  of  two-phase  currents,  as  a  rotary  converter, 
or  as  a  two-phase  synchronous  motor,  thus  including  all  the  im- 
portant elements  of  a  polyphase  system  except  the  induction  motor. 
It  has  already  been  stated  also  that  Bradley's  U.  S.  patent  No. 
409,450  fully  describes  a  three-phase  generator,  converter,  and 
synchronous  motor.  The  earliest  application  for  a  patent  by 
Tesla  setting  forth  a  three-phase  generator  and  motor  was  filed 
Oct.  12,  1887.  Even  after  the  polyphase  system  and  induction 
motor  had  been  made  known  to  the  public  during  1888  and  1889 
by  the  patents  and  papers  of  Ferraris,  Bradley,  and  Tesla,  it 
required  several  years  of  experiment  and  design  by  many  engineers, 
involving  much  labor  and  expense,  before  this  type  of  motor  became 
a  really  practical  success.  In  fact,  this  cannot  be  said  to  have 
occurred  until  about  1894.  Since  that  time  still  further  improve- 
ment has  been  made  in  efficiency,  higher  power  factor,  economy 
of  material,  etc.,  as  well  as  in  auxiliary  starting  and  regulating 
devices. 

No  one  is  directly  credited  with  having  invented  the  alternating- 
current  series  motor.  The  first  mention  of  the  possibility  of  such 
a  machine  was  apparently  made  by  Alexander  Siemens  before 
the  British  Institution  of  Electrical  Engineers  in  1884.  At  the  same 


130       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

time  he  indicated  the  advisability  of  laminating  the  entire  magnetic 
circuit.* 

For  several  years  after  that,  in  fact  until  about  1893,  *ne  simple 
a.  c.  series  motor  was  manufactured  quite  extensively  for  small 
fan  motors.  During  the  early  nineties,  among  others,  Rudolph 
Eichmeyer  and  Dr.  C.  P.  Steinmetz  experimented  with  large-size 
a.  c.  series  motors,  hoping  to  produce  a  desirable  single-phase 
railway  motor,  but  they  did  not  meet  with  much  success  on  account 
of  the  high  frequency  of  the  current  employed  by  them.  Interest 
in  this  type  of  motor  lapsed  thereafter,  on  account  of  the  develop- 
ment of  the  single-phase  induction  motor,  until  Mr.  Lamme  of  the 
Westinghouse  Company  produced  a  practical  machine  in  1902, 
upon  which  the  technical  press  and  engineers  again  became  much 
interested  in  the  development  of  the  machine,  and  many  modifica- 
tions to  improve  its  action  were  devised. 

The  invention  of  the  repulsion  motor  is  generally  credited  to 
Prof.  Elihu  Thomson,  who  discovered  the  physical  fact  that  any 
conducting  body  tends  to  be  repelled  by  a  magnet  excited  by  alter- 
nating current.  This  phenomenon  and  various  experiments  illus- 
trating it  are  described  by  him  in  a  paper  read  May  18,  1887,  before 
the  American  Institute  of  Electrical  Engineers  on  "Novel  Phe- 
nomena of  Alternating  Currents."  f  Professor  Thomson  in  this 
paper  also  showed  how  to  apply  the  principle  and  thus  obtain  a  new 
type  of  electric  motor.  In  this  machine  the  coils  are  short-circuited 
only  a  portion  of  the  time,  that  is,  while  moving  away  from  the 
poles.  The  form  of  motor  now  known  as  the  repulsion  type  em- 
bodies an  armature  having"  all  of  its  coils  short-circuited  and 
carrying  current  all  the  time  that  it  is  in  operation.  This  arrange- 
ment was  first  described  by  Professors  Anthony,  Jackson,  and 
Ryan  in  their  U.  S.  patent  No.  389,352,  issued  September  n,  1888, 
the  invention  having  been  made  in  1887.  Neither  of  these  forms 
of  repulsion  motor  was  considered  to  have  much  practical  impor- 
tance until  1902  or  later,  when  the  latter  was  experimentally  tried 
and  its  use  advocated  for  electric  railway  service  by  the  General 
Electric  Company  and  others. 

*  Journal  British  Institution  of  E.  E.,  p.  527,  Vol.  XIII,  1884. 
f  Transactions,  Vol.  IV,  p.  160. 

J  Papers  and  discussion  by  Slichter,  Steinmetz,  Blanck,  and  others  in  Trans.  Amer. 
Inst.  Elect.  Eng.,  Vol.  XXIII,  pp.  i-ioo,  January,  1904. 


CHAPTER    XIII. 

THE    SYNCHRONOUS   MOTOR. 

THE  synchronous  alternating-current  motor  is  merely  an  inverted 
alternator;  that  is,  the  same  machine  may  in  general  be  used  as  a 
generator  or  motor.  The  simplest  of  this  type  is  the  single-phase 
machine,  and  a  study  of  its  characteristics  will  also  explain  the  action 
of  the  corresponding  polyphase  motors.  For  example,  a  two-phase 
motor  may  be  regarded  as  a  combination  of  two  single-phase 
machines.  There  is,  however,  the  important  practical  fact  that  the 
single-phase  synchronous  motor  is  not  at  all  self-starting  (unless 
provided  with  special  starting  device),  while  the  polyphase  syn- 
chronous motor  is  self-starting  without  load,  in  which  limitations 
both  differ  from  the  majority  of  other  electric  motors.  It  should 
also  be  noted  that  this  type  of  motor  whether  single  or  polyphase 
requires  a  direct-current  supply  for  field  excitation. 

Not  Self -starting  for  Following  Reasons.  — The  rotation  of  the 
armature  of  a  direct-current  motor  is  due  to  the  fact  that  an  uni- 
directional torque  is  exerted  between  the  armature  and  field.  Any 
increase  in  load  tends  to  diminish  its  speed  and  counter e.m.f., which 
allows  a  greater  armature  current  to  flow,  producing  a  correspond- 
ing increment  in  the  driving  effort  of  the  motor. 

In  the  case  of  a  synchronous  motor  we  have  the  following  con- 
ditions: a  field  excited  by  direct  current  and  an  armature  supplied 
with  alternating  current.  The  first  condition  provides  a  field  of 
fixed  magnetic  polarity,  the  second  gives  an  armature  the  current  in 
which  is  alternating,  therefore  the  direction  of  rotation  also  tends 
to  be  variable.  (Fig.  73.)  If,  however,  the  motor  could  be  brought 
to  such  speed  (by  external  means)  that  the  half  of  the  armature 
represented  above  in  Fig.  73  would  be  below  after  the  current 
reversed,  a  fixed  polarity  in  space  would  result,  because  the  top  of 
the  armature  would  always  be  of  N  polarity  and  the  lower  part  of  S 
polarity,  producing  a  torque  in  one  direction  and  therefore  con- 
tinuous rotation.  For  this  to  occur,  the  armature  must,  however, 
revolve  one-half  a  turn  in  the  time  occupied  by  one  alternation  of 

131 


132      ELECTRIC  MOTORS,   THEIR  ACTION  AND  CONTROL. 

current,  and  a  full  turn  in  the  duration  of  one  cycle  of  current.  In 
other  words,  the  armature  must  generate  a  counter  e.m.f.  of  the 
same  frequency  as  the  applied  voltage.  When  this  condition  is  ful- 
filled the  motor  is  said  to  be  operating  in  synchronism,  hence  the 


FIG.    73.  —  VARIATION   OF   ARMATURE   POLARITY. 

term  synchronous  motor.  It  is  apparent  that  the  speed  of  this  motor 
must  remain  constant,  since  the  torque  must  be  unidirectional  to 
maintain  rotation,  so  that  the  speed  of  any  synchronous  motor  is  fixed 
by  its  number  of  poles  and  the  frequency  of  the  applied  voltage;  i.e., 

periods  per  sec.  X  60 

speed  in  r.p.m.  =  : (18) 

pairs  of  poles 

The  polyphase  synchronous  motor  is  self-starting  without  load, 
because  the  polyphase  currents  set  up  a  rotary  magnetic  field  in  the 
surface  of  the  armature,  which  reacts  upon  the  field  magnet  to  pro- 
duce mechanical  rotation.  The  circuit  of  the  field  winding,  however, 
must  be  open,  because  the  rotary  field  would  not  be  sufficiently  power- 
ful in  the  presence  of  the  field  flux.  After  the  rotary  member  is 
up  to  speed,  the  usual  d.  c.  field  excitation  is  established.  To 
prevent  excessive  voltage  being  generated  in  the  field  coils,  these  are 
separated  from  each  other  and  not  connected  together  in  series  until 
synchronous  speed  is  nearly  obtained.  It  is  also  desirable  when  thus 
starting  synchronous  motors  to  use  less  than  their  normal  working 
voltage. 

Action  of  Synchronous  Motor  under  Varying  Loads.  —  As  shown 
above,  the  motor  must  operate  at  a  definite  speed  on  a  circuit  of 
given  frequency,  or  not  at  all.  The  field  strength  being  also  con- 
stant, the  effective  counter  e.m.f.  of  the  motor  is  of  constant  value; 
therefore  it  would  seem  that  with  constant  applied  voltage,  which  is 
the  practical  condition,  the  armature  current  could  not  automat- 


SYNCHRONOUS  ALTERNATING-CURRENT  MOTOR. 


133 


ically  increase  to  enable  the  motor  to  exert  more  torque  in  order  to 
carry  additional  load.  The  peculiar  action  which  occurs  and  gives 
variable  torque  is  explained  as  follows:  Consider  two  ordinary 
single-phase  alternators  M  and  G  driven  by  independent  prime 
movers,  but  with  their  armatures  electrically  connected  in  series  and 
jointly  furnishing  power  to  an  external  load  Q  (Fig.  74).  The 


FIG.    74.  —  ALTERNATORS   CONNECTED   IN   SERIES. 

e.m.f.  of  the  system  (Er)  will  under  this  condition  be  Em  +  Eg  as 
shown  by  the  wave  and  vector  diagrams  A  and  B,  respectively 
(Fig.  76).  Since  there  is  inductance  in  the  armatures  and  line,  the 


FIG.    75,    A   AND 


—  ALTERNATORS   IN  SERIES  WAVE   AND  VECTOR  DIAGRAMS 
E.M.F. 'S   IN   PHASE. 


current  /  will  lag  behind  the  resultant  e.m.f.  or  Er  by  an  angle  6. 
If  the  load  should  change  or  the  steam  pressure  vary,  the  engine 
governors  would  tend  to  maintain  constant  speed;  but  one  would 
naturally  act  before  the  other.  This  difference,  however  small, 
would  cause  one  machine  and  its  e.m.f.  to  fall  behind,  so  that  its 
phase  angle  with  respect  to  the  current  would  be  less,  its  load  there- 
fore greater,  and  the  power  required  to  drive  it  correspondingly 


134       ELECTRIC  MOTORS,   THEIR  ACTION  AND   CONTROL. 

increased.  The  converse  is  true  of  the  other  alternator,  the  conse- 
quence being  that  the  former  continues  to  drop  back  in  phase  until 
it  is  about  180  degrees  behind  the  other  machine.  Referring  to 
the  vector  diagram  B  in  Fig.  76,  let  us  consider  exactly  what  occurs. 


FIG.   76. PHASE  RELATIONS  BETWEEN   ALTERNATORS  IN  SERIES,   SWINGING  OVER  TO 

PARALLEL   CONDITIONS. 


OEg  represents  the  e.m.f.  of  the  lagging  machine  and  OEm  that  of  the 
other,  so  that  OEr  is  the  new  resultant  e.m.f.  and  OI  the  new  current 
position,  which  maintains  the  same  phase  relation  with  Er  as  before, 
because  the  resistance  R  and  inductance  L  of  the  circuit  have  not 
changed.  Let  <j>m  and  <j>g  represent  the  new  phase  displacement  of 
Em  and  Eg  respectively  with  reference  to  the  current.  The  load  on 
each  of  the  alternators  is  now  EmI  cos  $m  and  Egl  cos  (f>g  respectively; 
the  angle  <f>0  being  less  than  (f)m,  the  load  Egl  cos  <j)g  on  the  engine 
that  drives  the  machine  G  is  greater  than  that  on  the  engine  driving 
the  machine  M,  hence  the  latter  tends  to  run  faster  while  G  will  fall 
off  in  speed.  The  load  on  G  increases  more  and  more  and  the 
angle  between  Em  and  /  becomes  greater  until  it  passes  through 
90  degrees,  after  which  cos  </>m  has  a  negative  value,  so  that  the  work 
done  by  alternator  M  is  negative,  that  is,  it  is  operating  as  a  motor, 
the  phase  relations  being  represented  in  Fig.  77.  Hence  the  opera- 
tion of  two  or  more  alternators  in  series  is  a  condition  of  unstable  equi- 
librium, unless  they  are  positively  connected  or  driven  from  the  same 
source  of  power  so  that  any  speed  change  is  common  to  both;  other- 


SYNCHRONOUS   ALTERNATING-CURRENT  MOTOR. 


135 


wise,  at  the  least  variation  in  load  or  speed,  they  will  instantly  fall 
out  of  step  and  tend  to  pass  into  the  condition  of  opposition,  paral- 
lelism or  1 80°  phase  relation. 

The  parallel  operation  of  two  or  more  alternators  is  a  stable 
one.  If  one  machine  tends  to  speed  up,  its  voltage  increases  and 
thus  it  would  supply  a  larger  current,  carry  a  heavier  load  and  be 
compelled  by  the  action  of  the  engine  governor  to  slow  down,  while 
the  under-loaded  machine  would  tend  to  speed  up,  thus  equalizing 


'VT? 


FIG.  77.  —  SYNCHRONOUS  MOTOR,  PHASE  RELATIONS  BETWEEN  LINE  VOLTAGE, 
CURRENT  AND   MOTOR  E.M.F. 

conditions.  This  statement  is  a  general  one  corroborated  by  the 
fact  that  a.  c.  generators  are  successfully  run  in  parallel  in  thousands 
of  commercial  plants.  On  the  other  hand  practical  difficulties  arise 
in  some  cases  of  parallel  operation,  because  of  slight  differences  in 
angular  velocities  due  to  "hunting"  of  governors,  which  may  throw 
the  machines  out  of  synchronism.  If,  after  the  alternator  M  reaches 
the  i8o-degree  phase  relation  with  respect  to  the  other  machine,  we 
disconnect  its  mechanical  driving  power,  it  would  continue  to  oper- 
ate as  a  motor,  provided  the  current  fed  to  its  armature  be  sufficient 
to  supply  copper  losses  and  stray  power  torque.  If  this  is  not  enough, 
armature  M  will  tend  to  lag  a  trifle,  causing  the  resultant  e.m.f. 
to  increase  so  that  more  current  flows.  This  increases  its  torque 
sufficiently  to  maintain  rotation,  unless  the  dropping  back  of  M 
should  be  so  great  in  duration  or  phase  that  the  synchronous  rela- 
tion is  broken. 

The  preceding  facts  may  be  summed  up  as  follows:  The  ability 
of  a  synchronous  motor  to  carry  a  variable  load  is  due  to  the  phase 
shifting  of  its  e.m.f.,  which  action,  taking  the  place  of  the  speed 


136       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

changes  of  other  types  of  motors,  alters  the  resultant  e.m.f.  so 
that  the  armature  current  automatically  adjusts  itself  to  the 
load. 

Starting  and  Synchronizing  of  Synchronous  Motors.  —  The  single- 
phase  synchronous  motor  not  being  self-starting,  as  already  ex- 
plained, requires  some  auxiliary  motor  to  bring  it  up  to  synchronous 
speed  and  into  proper  phase  relation  before  it  can  be  properly  con- 
nected to  the  supply  circuit.  Such  auxiliary  starting  device  may  be 
a  series,  repulsion  or  induction  (split-phase)  motor,  or  if  the  direct- 
current  field  exciter  is  large  enough,  it  may  be  used  as  the  starting 
motor,  being  supplied  with  current  from  a  storage  battery,  which  at 
that  time  also  furnishes  the  main-field  exciting  current.  Small  syn- 
chronous motors  are  often  constructed  with  the  starting  device  as 
an  integral  part  as  follows:  The  armature  core  is  provided  with  an 
additional  winding  and  commutator,  which,  connected  in  series 
with  an  extra  winding  on  the  field  cores,  makes  it  possible  to  start 
the  machine  as  a  series  motor.  The  commutated  armature  wind- 
ing is  connected  across  the  main  field  after  synchronous  speed  is 
attained  and  the  main  armature  is  connected  to  the  a.  c.  supply 
lines;  thus  the  machine  becomes  self-exciting. 

Polyphase  synchronous  motors  are  self-starting,  with  about  10 
to  15  per  cent  rated  load  torque,  through  the  development  of  a 
rotary  field  by  the  currents  in  the  armature  windings,  which,  acting 
upon  the  polar  faces,  drags  the  rotor  around.  To  start  in  this 
manner,  the  field  circuit  is  opened,  and  the  armature  supplied 
with  approximately  rated  load  current  at  about  one-half  rated 
voltage  through  a  transformer  or  compensator.  This  causes  the 
revolving  member  to  rotate  at  a  speed  approximating  that  of  syn- 
chronism. The  operator  then  closes  the  field  circuit,  thus  acceler- 
ating the  rotor  to  synchronous  speed,  after  which  the  armature  is 
supplied  with  current  at  rated  voltage  and  the  machine  is  ready  to 
carry  its  load.  This  method  of  starting  may,  however,  especially  if 
the  motor  be  large  with  respect  to  the  generators,  cause  serious 
voltage  fluctuations,  and  it  then  becomes  desirable  to  use  some 
other  starting  devices,  for  example,  an  auxiliary  machine  as  already 
stated  in  the  case  of  the  single-phase  synchronous  motor. 

It  is  impossible  with  ordinary  mechanical  speed-measuring 
instruments  to  determine  the  approach  to  synchronous  speed  as 
accurately  as  is  necessary  for  the  safe  connecting  of  synchronous 


SYNCHRONOUS  ALTERNATING-CURRENT  MOTOR. 


137 


motors  to  the  line.  Furthermore,  the  phase  relations  of  the  line 
and  motor  voltages  must  also  be  correct.  There  are,  however, 
simple  electrical  methods  of  determining  when  agreement  in  fre- 
quency with  proper  phase  relations  exists,  the  simplest  of  these  being 
the  lamp  methods,  one  of  which  follows:  In  Fig.  78  L  and  M 


FIG.  78.  —  CONNECTIONS  FOR  SYNCHRONIZING  SINGLE-PHASE  MOTOR. 
(LAMP  FILAMENTS  BLACK.) 


represent  respectively  single-phase  line  terminals  and  a  single- 
phase  synchronous  motor,  which  can  be  connected  by  the  double 
pole  switch  S.  Two  synchronizing  lamps  /  and  V  are  connected 
respectively,  as  shown,  across  the  two  gaps  in  the  circuit,  con- 
trolled by  the  switch  S.  By  means  of  the  auxiliary  starting  motor 
EF,  the  synchronous  machine  M  is  brought  to  approximately  the 
proper  speed.  With  the  rotor  member  of  the  motor  stationary, 
or  when  its  frequency  differs  greatly  from  that  of  the  circuit,  the 
alternations  of  current  and  the  corresponding  flickering  of  the 
lamps  /,  I'  are  too  rapid  for  the  eye  to  detect,  but  when  the  motor 
frequency  varies  only  slightly  from  that  of  the  line,  whether  higher 
or  lower,  the  synchronizing  lamps  will  glow  for  one  moment  and 
be  black  the  next.  The  smaller  the  difference  in  frequency  the 
less  rapid  the  flickering.  At  the  instant  that  the  voltages  are 
opposite  in  phase,  and  of  equal  value,  there  will  be  no  current 
through  the  lamps;  but  when  the  voltages  are  in  phase  their  full 
sum  is  applied  to  the  lamps,  which  then  glow  at  their  maximum 
brilliancy.  When  the  flashing  becomes  very  slow  the  motor  may 
be  connected  to  the  line  by  closing  the  switch  S  at  the  instant 
that  the  lamps  cease  to  glow.  If  the  motor  continues  to  operate 
properly,  its  field  strength  may  be  adjusted  so  that  the  line  current 
will  be  small  after  the  auxiliary  motor  or  starting-up  device  is 
disconnected.  It  is  better  to  connect  the  motor  to  the  line  as  it  ap- 
proaches exact  synchronism  rather  than  when  it  is  departing  from 


138       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

it;  that  is,  the  main  switch  S  should  be  closed  the  instant  the  lamps 
cease  to  give  light. 

Owing  to  the  fact  that  incandescent  lamps  do  not  glow  with  less 
than  30  or  40  per  cent  of  their  rated  voltage,  it  is  impossible  to 
determine  exactly  the  minimum  voltage  difference  which  is  the 
proper  condition  for  connecting  a  synchronous  machine,  and  thus 
the  current  passing  between  the  machines  and  the  line  may  be 
rather  high  upon  closing  of  switch  S.  To  avoid  the  danger  of 
such  rush  of  current,  the  lamps  may  be  diagonally  connected,  that 
is,  between  G  and  F  and  between  E  and  H  (Fig.  79),  in  which 


FIG.  79. CONNECTIONS  FOR  SYNCHRONIZING  SINGLE-PHASE  MOTOR  (MAXIMUM  LIGHT). 

case  they  glow  at  full  brilliancy  when  the  phase  relation  is  correct. 
The  lamps  may  also  be  replaced  by  voltmeters,  which  if  connected 
as  in  the  first  instance  would  indicate  zero  voltage  and  in  the  latter 
case  show  full  voltage. 

The  connections  of  the  synchronizing  lamps  for  a  three-phase 
circuit  are  similar  to  the  preceding,  but  three  lamps  are  employed 
as  shown  in  Fig.  80.  If  all  three  lamps  simultaneously  become 


FIG.    80.  —  CONNECTIONS   FOR   SYNCHRONIZING   THREE-PHASE   MOTOR. 

bright  or  dark,  the  connections  are  correct,  and  the  line  switch 
may  be  closed  at  the  instant  of  darkness.  It  may  happen,  how- 
ever, that  the  lamps  do  not  glow  at  the  same  instant  but  succes- 
sively. This  indicates  that  the  leads  are  not  connected  in  their 
proper  order.  In  this  case  the  motor  lines  should  be  transposed 
until  the  lamps  brighten  simultaneously.  After  the  machines  have 


SYNCHRONOUS  ALTERNATING-CURRENT  MOTOR.          139 

been  once  properly  connected  their  synchronizing  can  be  accom- 
plished with  a  single  lamp.  If  the  voltage  of  the  circuit  is  too  high 
for  the  direct  use  of  lamps,  transformers  should  be  inserted  as  indi- 
cated in  Fig.  81,  their  secondary  coils  being  in  series  with  each 


FIG.    8l.  —  CONNECTIONS    FOR   SYNCHRONIZING    ON    HIGH-VOLTAGE    CIRCUIT. 
EXCITER  USED    AS    STARTING    MOTOR. 

other  and  with  the  lamp  /.  The  latter  will  glow  when  the  motor 
e.m.f.  opposes  that  of  the  line,  provided  the  connections  of  either 
the  primary  or  secondary  coils  are  reversed,  the  former  case  being 
represented  in  Fig.  81. 

It  has  become  apparent  with  the  continual  increase  in  the  size  of 
units  that  better  means  of  synchronizing  than  those  afforded  by 
the  lamp  methods  just  described  were  desirable  because  serious 
trouble  ensues  if  large  machines  with  heavy  revolving  parts  are 
connected  together  when  not  exactly  in  step.  Such  a  device  is 
secured  in  the  synchronizer  or  "synchroscope."*  This  instrument 
consists  essentially  of  a  small  induction  motor,  of  which  the  fixed 
winding  or  stator  is  excited  from  the  line,  and  the  revolving  member 
or  rotor  is  supplied  with  current  (through  a  phase-splitting  device) 
from  the  machine  to  be  synchronized.  A  rotating  magnetic  field  is 
thus  set  up  in  the  synchronizer,  and  the  rotor  thereof  will  revolve  at  a 
speed  that  is  governed  by  the  difference  between  the  line  and  motor 
frequency.  The  shaft  of  the  rotor  is  provided  with  an  arm,  which, 
revolving  with  it,  serves  as  an  indicator.  When  the  frequencies  of 
the  currents  in  the  rotor  and  stator  of  the  synchronizer  are  the  same, 
the  magnetic  field  due  to  both  is  no  longer  a  rotary  one,  and  the  pointer 
remains  stationary.  This  condition  may,  however,  indicate  only 
an  equality  of  frequency,  not  necessarily  one  of  correctness  of  phase 
relation.  There  is  only  one  particular  position  of  rest  assumed 

*  Electric  Journal,  Vol.  I,  1904,  p.  692;  Vol.  IV,  1907,  p.  497. 


140       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

by  the  rotor  when  frequency  and  phase  agreement  both  exist;  and 
its  index  is  so  set  that  it  points  vertically  upward  when  these  con- 
ditions are  secured.  Accordingly,  while  agreement  in  frequency  is 
indicated  by  the  fact  that  the  index  of  the  synchroscope  is  stationary, 
the  angular  difference  between  such  direction  and  the  vertically 
upward  one  shows  the  phase  difference  existing.  Another  feature 
of  value  in  this  instrument  is  the  fact  that  it  also  shows  whether  the 
motor  (or  incoming  machine)  is  running  too  fast  or  too  slowly, 
because  if  running  too  fast  the  pointer  will  move  forward  in  a  clock- 
wise direction,  while  if  revolving  at  too  low  a  speed  the  pointer  will 
move  in  a  counter-clockwise  direction.  Thus  the  synchroscope 
accurately  indicates  frequency  and  phase  relations,  and  its  use  is  to 
be  recommended  in  connection  with  large  synchronous  motors. 

Phase  Relations  between  Constant  Line  Voltage  and  Motor  Current. 
—  The  synchronous  motor  in  practice  is  supplied  with  current  from 
a  constant-potential  a.  c.  circuit,  and  load  changes  cause  the  machine 


v/7 


f,  Motor  Lc&d 
4. 


FIG.   82,   A,   B   AND  C.  —  CHANGES  OF  CURRENT  VALUE   AND  PHASE  WITH  VARIATION  IN 
SYNCHRONOUS   MOTOR   LOAD. 

to  draw  currents  varying  not  only  in  value  but  also  in  phase  relation 
with  respect  to  the  line  voltage.  Fig.  82,  A,  B  and  c,  illustrates  the 
changes  in  current  value  and  phase  angle  which  occur  in  the  case  of 
a  3o-kw.  single-phase  synchronous  motor  when  the  load  is  5,  20, 
and  40  kw.  respectively,  motor  e.m  f.  and  line  voltage  being  equal 


SYNCHRONOUS  ALTERNATING-CURRENT  MOTOR.          141 

at  500  volts.  These  diagrams  show  that  the  angle  <j>0  between  the 
current  and  line  voltage  becomes  smaller  and  smaller  as  the  load  is 
increased,  and  would  at  about  45-kw.  load  reach  a  zero  value,  after 
which  upon  further  addition  of  load  it  becomes  greater  again,  in  the 
opposite  direction,  but  shifting  from  a  leading  to  a  lagging  angle. 

Change  of  load  is  not  the  only  way  to  produce  variations  in  the 
angular  relation  between  line  current  and  voltage;  it  may  be  caused 
by  adjusting  the  excitation  of  the  motor,  and  this  is  frequently  done 
in  practice  to  secure  a  leading  current. 

The  actions  occurring  in  a  synchronous  motor  with  variable  field 
excitation  can  readily  be  studied  by  means  of  vector  or  circle  dia- 
grams. Assume  for  example  any  condition  of  motor  load  and  let 
the  line  e.m.f.  be  represented  by  Eg,  the  current  by  /,  and  the  angle 
between  them  by  <j>g.  The  angle  d,  existing  between  the  resultant 
e.m.f.  Er  and  the  current  /  (which  depends  upon  the  values  of  fre- 
quency /,  inductance  L,  and  resistance  R  of  the  circuit),  is  theoreti- 
cally constant,  but  in  practice  it  varies  slightly,  since  the  permeability 
of  the  magnetic  circuit  is  only  approximately  constant.  Hence,  know- 
ing the  line  frequency  and  voltage  Eg,  the  load  amperes  /,  the  watts 
input  and  the  machine  constants  (L  and  R),  we  can  readily  draw 
vector  diagrams  representing  the  various  phase  relations  of  Eg,Em,Er, 
and  /  at  any  input,  or  any  mechanical  output,  if  we  recollect  that 
this  latter  is  equal  to  the  input  less  the  laRa  losses.  Formulae  can  be 
derived  from  these  vector  diagrams  by  which  any  one  of  the  various 
components  can  be  calculated  if  the  others  are  known  (pp.  150, 159). 

In  Fig.  83,  A,  lay  off  O-E^as  the  impressed  or  line  voltage,  OI  as 
the  current  at  an  angle  <t>0  from  Eg.  The  value  of  Er  is  proportional 
to  the  value  of  the  current  and  is  equal  to  I  VR2  -f  (2  7r/L)2,  while  the 
angle  6  is  cos"1  (R  +  VR2  +  (2  Tr/Z,)2) ;  hence  lay  off  this  value  of 
'Er  at  angle  0  ahead  of  /.  The  motor  e.m.f.  is  then  found  by  com- 
pleting the  parallelogram;  that  is,  it  is  parallel  and  equal  to  EgEr, 
its  true  vector  position  and  relative  value  being  OEm. 

The  power  input  of  the  motor  is  Egl  cos  <{>g.  The  motor  output, 
including  core  and  friction  losses,  is  E"gl  cos  <f>0  —  Ia2Ra  or  Eml  cos  <£m, 
wherein  Em  is  the  motor  e.m.f.  and  (/>m  angle  between  Em  and  /. 

With  the  line  voltage  Eg  maintained  constant,  the  current  /  may 
have  any  value  for  a  given  input,  depending  upon  the  value  of  cos  <f>0. 
For  Instance,  in  Fig.  83,  B,  let  the  motor  input  have  the  same  value  as 
in  Fig.  83,  A,  but  let  the  current  be  in  phase  with  the  line  e.m.f.; 


142       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 


-n? 


A,    B,    C   AND   D.  —  VARIATION    OF    POWER    FACTOR    (COS    <fo)    OF   CIRCUIT    BY 
CHANGE   OF   SYNCHRONOUS   MOTOR  E.M.F.     MAINTAINING   CONSTANT   INPUT. 


SYNCHRONOUS  ALTERNATING-CURRENT  MOTOR.          143 

that  is,  Egl  cos  fa  =  const,  and  fa  =  o.  The  new  position  of  £> 
(the  resultant  e.m.f.)  is  substantially  the  same  angle  ahead  of  the 
current  /,  since  R  and  L  have  not  changed  materially,  but  Er  is 
smaller  than  before  because  /  is  less  for  the  same  load.  The  new 
value  of  Em  (the  motor  e.m.f.)  is  obtained  by  completing  the  paral- 
lelogram of  which  OEg  is  one  side  and  EgEr  another.  This  new 
position  and  value  of  Em  are  shown  by  the  line  OEm,  which  is  longer 
than  in  the  preceding  case.  This  increase  can  be  brought  about 
only  in  one  way,  i.e.,  by  increasing  the  strength  of  the  motor  field, 
because  the  speed  is  synchronous  and  therefore  constant. 

In  Fig.  83,  c,  let  us  assume  a  motor  load  of  the  same  amount  as  in 
the  two  preceding  instances,  but  with  the  current  leading  the  line 
e.m.f.  by  an  angle  fa  equal  to  the  lag  in  the  first  case  (Fig.  83,  A). 
The  angle  0  between  Er  and  /  remains  practically  constant  because 
R  has  not  changed  and  L  only  slightly.  Lay  off  OEr  as  the  resultant 
e.m.f.  an  angle  0  ahead  of  OI.  Completing  the  parallelogram  we 
have  OEm  representing  the  phase  and  value  of  the  motor  e.m.f. 
that  corresponds  to  these  new  conditions.  An  inspection  of  this 
diagram  shows  that  the  motor  e.m.f.  (Em)  is  now  of  still  larger  value. 
In  fact  the  field  strength  of  the  synchronous  motor  can  be  increased 
so  much  that  the  motor  e.m.f.  is  considerably  greater  than  the  line 
e.m.f.,  the  result  being  that  the  angle  fa  becomes  a  large  leading  one; 
which  condition  is  shown  in  Fig.  83,  D.  Hence,  if  the  motor  field  is 
gradually  strengthened  the  line  current  can  be  made  ultimately  to 
lead  the  line  e.m.f.  This  phenomenon  of  the  synchronous  motor 
is  of  value  in  the  transmission  of  power,  since  a  super-excited  motor 
can  be  employed  to  raise  the  power-factor  of  the  circuit,  which  usually 
tends  to  have  a  lagging  current. 

Torque  Conditions  of  Synchronous  Motor  depending  upon  Angle 
between  Current  and  Motor  E.M.F.  —  The  current  flowing  in  the 
armature  of  a  synchronous  motor  may  have  one  of  three  general 
phase  relations  with  respect  to  the  motor  e.m.f.  or  Em.  This 
phase  angle  may  be  180  degrees  (Fig.  84,  A)  because  motor  action  is 
here  considered,  the  same  relation  existing  in  a  d.  c.  motor;  it  may 
be  less  than  180  degrees  (Fig.  84,  B)  or  it  may  be  more  than  180 
degrees  (Fig.  84,  c). 

The  most  efficient  condition  for  motor  output  exists  in  the  first 
case  when  the  phase  angle  between  Em  and  Ia  is  180  degrees,  since 
then  the  current  required  to  produce  the  desired  torque  is  a  mini- 


144       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

mum.  This  may  be  proven  as  follows:  In  Fig.  85,  A,  the  wave 
diagrams  of  current  and  e.m.f.  are  shown  with  a  phase  displace- 
ment of  1 80  degrees;  the  resulting  power  curve  P  being  negative  at 
every  instant;  thus  for  a  given  area  (representing  motor  power)  the 


FIG.  84,  A,  B,  AND  C. VARIOUS  PHASE  RELATIONS  OF  SYNCHRONOUS  MOTOR  E.M.F. 

AND  CURRENT  WITH  CONSTANT  OUTPUT. 

value  of  I  will  be  ?  minimum.  This  condition  is  also  shown  by 
the  equation  End  cos  (j>m  =  motor  power;  because  cos  $m  has  its 
maximum  negative  value  =  -  i,  when  <j>m=  180  degrees;  hence  to 
produce  a  given  power  with  Em  constant,  /  will  have  minimum 
value.  In  Fig.  85,  B,  the  wave  diagram  shows  the  current  displaced 
less  than  180  degrees  with  respect  to  motor  e.m.f.,  in  which  case 
the  resulting  power  wave  has  both  negative  and  positive  values; 
hence  with  a  given  current  the  motor  power  represented  by  the 
negative  area  is  not  only  smaller  than  in  the  preceding  case  but  is 
still  further  diminished  by  the  positive  area,  so  that  the  available 


SYNCHRONOUS  ALTERNATING-CURRENT  MOTOR. 


145 


motor  power  is  considerably  less  than  when  the  current  and  e.m.f. 
differ  by  180  degrees.  Therefore,  to  have  the  same  power  the 
current  must  be  greater  in  the  second  case  (Fig.  85,  B).  Evidently 
similar  conclusions  apply  when  the  current  leads  the  motor  e.m.f. 


FIG.    85,    A,    B    AND    C.  —  POWER   CURVES   WITH    <j)m  = ,    <,    >   l8oc. 

as  shown  in  Fig.  85,  c.  There  is  also  another  effect  when  the  cur- 
rent and  motor  e.m.f.  differ  less  or  more  than  180  degrees  in  phase; 
namely  a  strengthening  or  weakening  of  the  motor  field.  For 
example,  if  the  phase  displacement  between  current  and  motor 


146       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

e.m.f.  is  1 80  degrees,  the  current  reaches  its  maximum  at  the  same 
instant  as  the  e.m.f.,  which  condition  is  represented  by  the  position 
of  the  armature  in  Fig.  86.  In  this  case  the  armature  current 


FIG.  86.  —  ANGLE  BETWEEN  Em  AND  LINE  CURRENT  IS  l8o°  DISTORTION  OF  MAIN  FIELD. 

neither  magnetizes  nor  demagnetizes  the  field,  with  the  moderate 
flux  densities  adopted  for  a.  c.  machinery.  The  effect  is  merely  to 
distort  the  field,  since  the  N  poles  of  the  armature  increase  the  flux 
at  the  S  poles  of  the  field  and  diminish  it  equally  at  the  N  field 
poles  —  a  similar  balanced  effect  being  produced  by  the  S  arma- 
ture poles. 

If  the  armature  current  lags  less  than  180  degrees  behind  the 
motor  e.m.f.  it  reaches  its  maximum  value  at  the  instant  indicated 
in  Fig.  87.  An  inspection  of  this  diagram  shows  that  the  field 


FK5.    87.  —  ANGLE    BETWEEN   Em   AND    LINE    CURRENT    LESS    THAN    l8o°; 
MOTOR   FIELD    STRENGTHENED. 

strength  of  the  motor  is  increased  because  the  flux  direction  of  the 
armature  favors  that  of  the  field. 

If  the  armature  current  leads  the  motor  e.m.f.,  that  is,  the  phase 
difference  is  more  than  180  degrees,  then  this  current  attains  its 
maximum  before  the  armature  reaches  the  position  of  maximum 
e.m.f.  as  shown  in  Fig.  88.  The  result  with  this  phase  relation  is  a 


SYNCHRONOUS   ALTERNATING-CURRENT  MOTOR.          147 

weakening  of  the  main  field  due  to  the  opposition  of  the  armature 
magnetization,  like  poles  being  contiguous. 

A  summary  of  the  preceding  facts  is  as  follows. 

i.  For  a  given  current  a  motor  develops  maximum  torque  when 
the  phase  angle  between  its  e.m.f.  (Em)  and  the  armature  current 
(/)  is  1 80  degrees. 


FIG.  88.— ANGLE   BETWEEN    Em   AND   LINE   CURRENT   MORE   THAN    l8o°; 
MOTOR   FIELD    WEAKENED. 

2.  When  Em  and  /  'differ  in  phase  by  more  or  less  than   180 
degrees  the  torque  for  a  given  value  of  /  is  less  than  when  the  phase 
angle  is  180  degrees. 

3.  When  the  phase  angle  <j>m  is  180  degrees  the  armature  reac- 
tion merely  distorts  the  field,  because  one  pole  tip  is  strengthened 
as  much  as  the  other  is  weakened,  with  ordinary  flux  density. 

4.  When  the  angle  <}>m  is  less  than   180  degrees  (line  current 
lagging  with   respect   to   Em)    the   armature   reaction   strengthens 
motor  field. 

5.  When  the  angle  <j>m  is  more  than  180  degrees  (/  leading  Em) 
the  armature  reaction  weakens  motor  field. 

6.  The  condition  of  lagging  current  with  respect  to  motor  e.m.f. 
($  <  1 80  degrees)   gives  more  stable  operation  than  the  others 
because  the  increase  of  field  strength  augments  the  torque  of  the 
motor,  tending  thus  to  hold  it  in  step. 

Conditions  of  Maximum  Output. — Various  relations  between  the 
maximum  motor  output,  line  voltage  Eg,  motor  constants  R,  L, 
and  /,  the  motor  current,  can  be  calculated.  They  are  only  of 
theoretical  interest,  however,  and  that  of  a  low  order,  because  the 
motor  windings  would  be  destroyed  long  before  these  conditions 
could  be  reached. 

For  example,  a  typical  i38-kw.  synchronous  motor  has  an  arma- 


148       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

ture  resistance  of  0.13  ohm,  the  voltage  across  the  terminals  is  1443 
volts,  and  for  theoretical  maximum  output  the  current  would  be 

/  = =  - — -  =5550  amperes,  which  is  about  55  times  its  rated 

2  R      0.26 

load  current  of  102  amperes,  the  heating  effect  being  552  =  3025 
times  the  normal.  The  corresponding  values  of  maximum  watts 
output  and  voltage  have  also  been  calculated,  but  the  above  pre- 
posterous result  demonstrates  that  little  time  should  be  spent  in 
determining  or  discussing  such  imaginary  conditions. 

Operative  Limits  of  Synchronous  Motor.  — The  rotation  of  a 
synchronous  motor,  as  already  explained,  is  absolutely  dependent 
upon  the  maintenance  of  synchronous  relation  with  the  line  voltage. 
Hence  the  driving  power  is  exerted  through  a  relatively  easily 
broken  link,  somewhat  flexible  it  is  true,  but  not  so  strong  as  that 
of  the  series,  shunt,  or  induction  motors.  These  gradually  drop  in 
speed  and  finally  become  stalled  upon  application  of  excessive 
overload;  wheieas  the  stoppage  of  a  synchronous  motor  is  very 
sudden. 

This  abrupt  stopping  of  a  synchronous  motor  is  due  to  the  very 
fact  upon  which  the  ability  of  the  machine  to  carry  variable  loads 
depends;  namely,  the  phase  swing  of  the  armature  e.m.f.  with 
respect  to  the  line  voltage  as  load  comes  on,  thus  increasing  the 
resultant  pressure  and  armature  current.  This  retardation  of  the 
motor  armature  cannot  indefinitely  increase  its  driving  power, 
because  ultimately  the  phase  angle  between  motor  voltage  and 
current  approaches  90  degrees.  At  this  point  the  power  falls  to 
zero  (End  cos  $m  =  o)  and  the  machine  stops,  if  it  has  not  already 
done  so  before  the  go-degree  limit  is  reached.  Practically,  the 
stopping  would  occur  before  (/>m  is  reduced  to  90  degrees,  because 
the  driving  power  must  be  sufficient  to  overcome  the  core  losses, 
windage,  friction,  etc.,  plus  any  external  load. 

Investigation  of  the  operative  range  curves  (Fig.  89)  of  the  syn- 
chronous motor  indicates  that  it  has  two  conditions  of  operation, 
namely,  a  stable  and  an  unstable  one.  The  unstable  condition  of 
operation  exists  when  the  phase  angle  between  armature  and  line 
voltages  is  less  than  a  certain  value,  ranging  between  100  degrees 
and  120  degrees,  depending  upon  the  resistance  and  reactance  of 
the  motor  armature.  Thus,  if  the  motor  should  be  operating  on 
the  unstable  portion  of  the  curve,  that  is,  between  zero  and  about 


SYNCHRONOUS  ALTERNATING-CURRENT  MOTOR. 


149 


no  degrees,  any  attempt  to  increase  the  load  would  be  accompanied 
by  retardation  of  the  armature,  which  would  not,  however,  augment 
the  driving  power  of  the  machine;  with  the  result  that  the  synchro- 
nous link  is  broken  and  rotation  ceases.  Conversely,  while  operating 


FIG.    89.  —  OPERATIVE-RANGE    CURVES    OF    A    3O-KW.    5OO-VOLT 
SINGLE-PHASE    SYNCHRONOUS    MOTOR. 

on  this  unstable  portion  of  the  curve,  if  the  motor  load  be  decreased, 
acceleration  of  the  armature  results,  which  augments  the  driving 
power  of  the  machine,  and  acceleration  is  continued  until  the  crest 
of  the  power  curve  is  passed  and  the  stable  condition  reached. 


150       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

The  stable  condition  of  operation  for  synchronous  motors  exists 
when  addition  of  load  causes  a  retardation  of  the  armature  with  in- 
crease oj  driving  power  until  load  and  driving  power  balance.  If, 
however,  it  be  attempted  to  overload  the  motor  considerably,  the 
resulting  retardation  of  the  armature  causes  the  angle  between  the 
motor  e.m.f.  and  line  voltage  to  decrease  so  that  it  is  less  than 
the  value  corresponding  to  that  of  maximum  driving  effort;  therefore 
the  motor  passes  into  the  unstable  condition  of  operation  and  stops. 
If  the  load  on  the  motor  be  decreased  while  the  motor  is  operating 
on  the  stable  portion  of  the  curve,  the  armature  is  accelerated  and 
the  driving  power  decreased  until  a  balance  is  obtained. 

The  range  of  driving  power,  and  thus  the  capacity  of  a  given 
synchronous  motor,  depend  upon  its  field  excitation  and  armature 
constants  (resistance  and  reactance  of  its  winding).  The  driving 
power  of  such  a  machine  is  generally  greater  the  stronger  its  field, 
within  the  limits  attainable  in  practice.  For  different  machines, 
other  things  being  equal,  that  one  having  the  larger  impedance  angle 
(tan"1  2  TT/Z,  -T-  R)  has  the  higher  overload  capacity  or  greater 
stability. 

The  approximate  operative  range  of  any  synchronous  motor  for 
various  conditions  of  excitation  can  be  predetermined  if  the  line 


FIG.    90. GENERAL    VECTOR    RELATIONS    OF    LINE    VOLTAGE,    CURRENT 

AND   MOTOR   E.M.F. 


voltage,  armature  e.m.f.,  resistance,  inductance  and  frequency  are 
known.  The  equations  for  this  calculation  are  obtained  from  the 
ordinary  diagram  showing  the  vector  or  space  relations  between  the 
line  voltage,  current  and  armature  e.m.f.  Inspection  of  Fig.  90 
shows  that  the  resultant  voltage 


Er  =  VEm2  +  Eg2  +  2  EmEg  cos  d,  (19) 


SYNCHRONOUS  ALTERNATING-CURRENT  MOTOR.          151 

where  d  is  the  angular  relation  between  motor  and  line  voltages. 
From  this  value  the  armature  current  can  be  obtained  by  dividing  the 
resultant  voltage  by  the  armature  impedance,  thus: 


_  _  _         VEm2    +   Eg2    +    2  EmEg  COS  d  ,       , 

1    =  tLr  -f-  Z   =  -  --  (2O) 

(27T/Z,)2 


The  driving  power  of  the  motor,  or  that  portion  of  the  input  con- 
verted into  mechanical  power,  is 

Wm   =  Eml  COS  <£m, 

<j)m  being  the  angle  between  motor  voltage  and  line  current.  The 
value  of  this  angle  <£m  depends  upon  the  phase  relation  between 
line  voltage  and  armature  e.m.f.,  the  value  of  these  voltages  and  the 
constants  of  the  armature  circuit  or 


,              .                            _t  Eg  +  Em  COS  d 
<t>m   =  d  +  d  -  COS    1 (2l) 


Thus 

Wm   = 


(22) 

where  6  is  the  impedance  angle  of  the  motor  armature.  These 
equations  will  now  be  applied  to  the  determination  of  the  character- 
istic curves  of  the  following  synchronous  motor : 

Rated  capacity 40  h.p.  or  30  kw. 

Speed 900  r.p.m. 

Poles 8 

Frequency  (/) 60  periods  per  sec. 

Line  voltage  (Eg) 500 

Armature  resistance   (R) 0.8  ohm 

Armature  reactance  at  60  p.p.s.  (X) 2.5  ohms 

Armature  impedance  at  60  p.p.s.  (Z) 2.63  ohms 

Impedance  angle  of  motor  armature  (9) 71°  30' 

Stray  power  losses  at  500  volts  excitation  and  900  r.p.m 2.40  kw. 

Let  us  for  example  determine  the  various  values  of  armature 
current  (/) ,  of  motor  input  Wg,  and  of  motor  output  Wm,  when  both 
line  (Eg)  and  motor  (Em)  voltages  are  500  volts,  and  the  phase 
angle  between  Eg  and  Em  altered  from  80  degrees  to  180  degrees  in 


152       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 


steps  of  20  degrees.     Proceeding  for  d  =  80  degrees  and  Eg  =  Em  = 
500  volts,  we  have  from  equation  (19)  p.  150, 


Er  =  V(5oo)2  +  (soo)2  +  2  (500)  (500)  cos  80°  =  766  volts. 
From  eq.  (20),  /  =  Er  -r-  z  =  766  -r-  2.63  =  291  amps. 

i  <; oo  +  500  cos  80° 
Fromeq.(2i),^m=8o°  +  7i°30/-cos-1    ^-— =ui°3o/. 

COS  (f>m=    —   .368. 

From  eq.  (22), 

Wm  or  motor  output  =  500  X  —  291  X  368  =  -  53.5  kw. 
</>o=  $m  -  d  =  in0  30'  -  80°  =  3 1°  30';  cos  0,  =+  .852. 
Wg=*  500  X  291  X  .852  =  124  kw. 

The  same  steps  could  be  followed  out  in  deriving  corresponding 
values  of  Er,  /,  Wm,  etc.,  when  d  is  varied.  Collecting  the  results  of 
such  calculations  and  tabulating  them  we  have: 


9. 

fin 

volts. 

I-Er 

L~  z' 

0m- 

COS  0m. 

Wm,  kw. 

00- 

COS  00. 

Wg,  kw. 

amps. 

80° 

766 

291 

in0  30' 

-.368 

-53-5 

3i°  30' 

.852 

124.0 

100° 

643 

245 

122° 

-•530 

-66.0 

22° 

.927 

"5-3 

120° 

500 

190 

I32° 

-.669 

-63-5 

12° 

.978 

93-o 

I40° 

350 

128 

I42° 

-.783 

-52.5 

2° 

•999 

64.4 

160° 

175 

66 

152° 

-.883 

—  29.6 

-8° 

.990 

32-9 

1  80° 

o 

o 

o 

A  curve  plotted  between  the  listed  values  of  d°  and  Wm  as  abscissae 
and  ordinates  respectively  gives  the  5oo-volt  load-range  curve  shown 
in  Fig.  89.  The  corresponding  curves  for  300,  400,  600,  and  700 
volts  indicated  in  the  same  figure  could  be  obtained  in  the  same 
manner,  though  for  convenience  the  authors  employed  the  circle 
diagrams  shown  later,  on  pages  155,  156. 

It  is  to  be  noted  that  the  possible  load  capacity  of  this  machine 
is  greatest  at  the  highest  field  excitation  .considered;  but  at  these 
heavier  loads  the  armature  current  is  excessive,  as  shown  in  Fig.  95. 
When  excited  so  that  its  armature  produces  500  volts  counter  e.m.f. 
the  motor  has  about  80  per  cent  overload  capacity  before  instability 
is  reached.  At  greater  excitations,  for  example  at  600  volts,  the 


SYNCHRONOUS  ALTERNATING-CURRENT  MOTOR.          153 


320 


280 


200 


I" 


\  \ 


Degrees 


/so 


FIG.  91.  —  "CURRENT-PHASE  SWING  ANGLE"  CURVES  OF  30-KW.  SINGLE-PHASE  SYN- 
CHRONOUS MOTOR. 

crest  of  the  power  curve  occurs  at  140  per  cent  overload.  The  im- 
portant fact  follows  that  with  a  synchronous  motor  liable  to  be 
subjected  to  widely  variable  loads,  greater  stability  is  obtained  by  ad- 
justing its  field  excitation  so  that  the  armature  e.m.f.  is  equal  to  or 
somewhat  greater  than  the  line  voltage.  Increase  of  armature  volt- 
age should  not,  however,  be  carried  very  high,  since  the  current 
for  a  given  load  thereby  becomes  too  great,  causing  excessive  heating 


154       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

of  the  machine.  For  example,  the  armature  current  for  30  kw.  at 
500  volts  is  70  amperes;  at  600  volts,  82  amperes;  and  at  700  volts, 
1 20  amperes.  The  curves  between  current  and  phase  angle  given 
in  Fig.  91  are  derived  from  the  values  of  d  and  I  in  the  table  just 
preceding.  These  show  that  as  the  angle  between  line  e.m.f.  and 
motor  e.m.f.  becomes  smaller  the  current  increases  rapidly. 

Circle  Diagrams  of  Synchronous  Motor.  —  If  we  consider  a  given 
line  voltage  and  armature  e.m.f.  of  a  synchronous  motor,  and  plot 
the  vector  positions  and  relative  values  of  line  voltage,  armature 
e.m.f.,  resultant  voltage  and  current  through  a  phase  swing  of  180 
degrees,  it  is  found  that  while  naturally  the  locus  of  the  motor  volt- 
age is  a  circle,  the  loci  of  the  resultant  voltage  and  armature  current 
are  also  circles.  The  centers  of  these  circles  are  at  different  points. 
These  circle  diagrams  of  the  synchronous  motor  are  useful,  and  by 
their  application  the  values  of  Er,  /,  and  <j>m  can  be  directly  deter- 
mined and  thus  the  power  or  operative  load  range  curves  of  the  motor 
obtained  without  the  lengthy  calculations  based  upon  the  preced- 
ing equations.  One  set  of  circle  diagrams  is  required  for  each 
motor  excitation.  Their  construction  and  application  are  as  follows : 
Lay  off  in  a  horizontal  direction  and  to  scale  the  line  OEg  (Fig.  92) 
representing  the  line  voltage,  then  add  to  it,  in  the  same  direction, 
the  line  EgEmf  which  corresponds  with  and  is  proportional  to  the 
motor  e.m.f.;  also,  lay  off  to  the  left  of  Eg  a  distance  proportional 
to  and  representing  the  motor  voltage  as  above.  Then  with  Eg  as 
a  center  and  EgEm'  as  a  radius  describe  a  circle.  This  is  the 
locus  of  the  resultant  voltage  Er.  Its  maximum  value  is  propor- 
tional to  the  distance  from  O,  through  Eg  to  Em,  while  its  minimum 
value  is  the  distance  along  the  same  diameter  from  O  to  the  point 
at  which  this  diameter  cuts  the  left-hand  side  of  the  circumference; 
that  is,  the  length  OQ.  With  the  point  O  as  a  center  describe  a 
circle  having  a  radius  representing  and  proportional  to  Em.  This  is 
the  locus  of  the  motor  voltage.  Through  the  point  O  draw  the  line 

OK  so  that  it  makes  an  angle  6  ( the  impedance  angle  =  tan"1  — —  J 

with  the  horizontal  line  OEg.  The  location  of  the  point  K  is  ob- 
tained by  using  the  point  O  as  a  center  and  a  radius  equal  to  OEfm\ 
that  is,  OK  is  equal  to  OEm'.  Then  from  the  point  K  along  the 
line  OK  towards  O  lay  off  a  distance  equal  to  Em  and  with  this  new 
point  P  as  a  center  and  PK  as  a  radius  describe  a  circle.  This  repre- 


SYNCHRONOUS  ALTERNATING-CURRENT  MOTOR.          155 

sents  the  locus  of  the  current  vector  and  is  proportional  thereto, 
being  the  impedance  drop,  which  is  numerically,  for  any  vector 
position,  equal  to  the  amperes  multiplied  by  the  impedance  Z.  The 
maximum  current  position  is  represented  by  the  line  OK,  which  is 
numerically  equal  to  OK  -f-  Z,  being  obtained  when  the  motor 
voltage  and  line  voltage  are  in  phase  (i.e.,  360  degrees  displacement 
existing  between  them).  The  minimum  value  of  the  current  is  the 
distance  OL  divided  by  the  impedance,  and  is  obtained  when  the 
motor  voltage  is  180  degrees  from  the  line  voltage.  Three  sets  of 
circle  diagrams  are  given.  The  first  of  these  (Fig.  92)  shows  the 


FIG.    92.  —  CIRCLE    DIAGRAM    OF    SYNCHRONOUS    MOTOR;    Eg  =  500    VOLTS, 
Em  =30O  VOLTS,    6  =  60°. 

loci  of  resultant  voltage,  motor  voltage  and  current,  when  the  motor 
voltage  is  300  and  the  line  voltage  500  volts.  The  second  circle  dia- 
gram (Fig.  93)  illustrates  the  corresponding  loci  when  motor  and  line 
e.m.f.  are  each  500  volts;  and  the  third  diagram  (Fig.  94)  sets  forth  the 
loci  when  the  motor  voltage  is  700  and  the  line  voltage  500  volts.  Con- 
sider the  first  diagram,  and  let  it  be  desired  to  obtain  the  resultant 
voltage,  motor  current,  value  of  (f>m,  and  driving  power  of  motor  when 
the  angle  between  the  motor  voltage  and  the  line  voltage  is  130 
degrees.  The  procedure  is  as  follows:  From  the  point  O  draw  the 
line  OEm  so  that  it  makes  an  angle  of  130  degrees  with  OEg.  This 
represents  the  angular  position  of  the  motor  voltage  with  respect  to 
the  line  voltage.  Then  with  the  point  Em  as  a  center  and  a  radius 
equal  to  OEg  describe  an  arc  which  cuts  the  resultant  voltage  vector 
locus  at  the  point  Er.  OEr  represents  the  scale  value  (=  375  v.)  of 
the  resultant  voltage  and  its  correct  angular  position.  With  O  as  a 
center  and  with  the  distance  OEr  as  a  radius  describe  an  arc  inter- 


156       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

^  '  K 


FIG.  93.  —  CIRCLE   DIAGRAM   OF   SYNCHRONOUS   MOTOR J 
Eg  =  500   VOLTS,    Em  =  500   VOLTS,    6  =  6o°. 


FIG.    94.  —  CIRCLE    DIAGRAM    OF    SYNCHRONOUS    MOTOR  J    Eg 
Em  —  700  VOLTS,    6  =  60°. 


5OO    VOLTS, 


seating  the  current  vector  locus  at  the  point  /.  The  line  OI  repre- 
sents the  true  vectorial  position  of  the  armature  current,  and  if  its 
scale  value  be  divided  by  the  impedance,  the  correct  value  of  this 
current  is  given  in  amperes  (||f  =  146).  The  angle  contained 
between  EmOI  is  the  angle  <f>m,  and  its  value  (166  degrees)  can  be 
measured  on  the  motcr  voltage  circle.  The  product  then  of  Eml 


SYNCHRONOUS  ALTERNATING-CURRENT  MOTOR. 


157 


cos  (f)m  gives  the  motor  driving   power,  or   300  X  146  X  -  0.97 
=  —  42.5  kw. 

Various  angular  positions  between  Eg  and  Em  can  similarly  be 
assumed,   the  motor   current   and    driving   power   determined   as 


/30 


FIG.   95.  —  CURRENT-POWER  CURVES,   3O-KW.   SYNCHRONOUS  MOTOR. 

shown,  and  these  different  values  plotted  as  ordinates  with  the 
phase  angle  between  Eg  and  Em  as  abscissae;  the  result  being  the 
motor  "  load-range  curves"  and  "current  curves"  shown  in  Figs. 
89,  90  and  95,  respectively,  of  which  the  500-  volt  series  were  obtained 
by  calculation,  using  equations  (19)  to  (22).  It  can  very  readily 


158       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

be  seen  by  means  of  the  operative  range  curves  what  angle  between 
motor  voltage  and  line  voltage  represents  the  pulling  out  or  stop- 
ping condition.  This  occurs  with  the  typical  40-h.p.  motor  ex- 
amined when  the  angle  EmOI  is  a  little  less  than  no  degrees. 

V  Curves  of  the  Synchronous  Motor.  — All  the  working  charac- 
teristics of  a  synchronous  motor  can  be  determined  from  its  opera- 
tive load  range  curves,  which  were  derived  and  discussed  in  the 
preceding  pages.  For  example,  the  current- power  curves  are 
obtained  by  plotting  curves,  having  corresponding  current  values  as 
ordinates  and  kilowatts  as  abscissae.  Two  sets  of  current-power 
curves  exist  (Fig.  95);  one  group  showing  the  relation  between 
current  and  output  or  driving  power,  and  the  other  between  current 
and  armature  input.  The  armature  watts  input  are  determined 
by  adding  the  corresponding  PRa  losses  and  watts  output. 

The  characteristic  V  curves  of  the  synchronous  motor  are  readily 
determined  from  the  current-power  input  curves,  by  proceeding  as 
follows:  Assume  any  constant  input,  say  3o-kw.,  then  draw  the 
straight  line  AB  vertically  through  the  3o-kw.  abscissa  point  of  the 
current-power  curve  (see  Fig.  95),  and  from  the  intersection  of 
the  line  AB  with  the  different  input  curves  we  can  determine  the 
various  armature  currents.  For  example,  at  300  volts  this  current 
is  93  amps.,  at  400  volts  it  is  68  amps.,  at  500  volts  60  amps.,  etc. 
The  relation  existing  between  the  motor  e.m.f.'s  and  these  current 
values  used  as  abscissae  and  ordinates,  respectively,  gives  the  3O-kw. 
current  V  curve  for  the  motor  of  the  text.  The  complete  series 
of  these  V  curves  given  in  Fig.  96  were  obtained  by  following  a 
like  procedure  for  the  different  values  of  input  employed. 

A  study  of  these  curves  indicates  that  at  any  given  input,  as  the 
motor  voltage  is  increased  the  armature  current  decreases,  until 
a  minimum  value  is  reached,  after  which,  upon  further  strengthen- 
ing of  the  motor  field,  the  current  begins  to  increase.  The  varia- 
tion in  the  armature  current  for  a  given  range  of  field  excitation  is 
greater  at  the  lighter  loads,  and  the  curve  obtained  when  the  motor 
is  running  practically  without  load  is  of  substantially  a  V  shape, 
hence  the  name;  while  at  the  higher  loads  the  curve  flattens  out 
considerably,  appearing  like  an  arc  of  a  large  circle  at  rated  load 
input  and  beyond.  These  same  V  curves  can  be  determined 
by  calculation,  employing  a  formula  which  is  derived  from  the 
simple  vector  relations  existing  between  line  volts,  motor  volts, 


SYNCHRONOUS  ALTERNATING-CURRENT   MOTOR.          159 


Motor  £/>?./:' 

FIG.    96. —  CURVES  OF  3O-KW.   SYNCHRONOUS  MOTOR. 

and  current  in  the  case  of  a  synchronous  motor,  this  equation 
being 

Em  =  V(E,  cos  0,  -  7£a)2+  (Eg  sin  0,  -  IX)2  *,         (23) 
but  its  use  is  rather  tedious. 


*  From  vector  diagram,  Fig.  90,  it  is  apparent  that 

Em2=Eg2+Er2-  2EgErCOS(d- 

wherein  E^  =  Eg9  (cos2  <fo  +  sin2  <f>g). 


(A) 


>      '-"Tl-^f 

cos  (0  —  00)  =  cos  0  cos  00  +  sin  0  sin  00  ==  —  cos  00  4-  —  sin  00. 

/>  /v 

Substituting  the  above  for  their  equivalent  terms  in  equation  (A)  we  have 
En2  =  Eg2  cos2  00  —  2  7J?£0  cos  00  +  PR2  +  Eg2  sin2  00—2  7X£0  sin  00  + 
=  (Eg  cos  00  -  IR)2  -f-  (£0  sin  00  -  IX}2 


or       Em  =  V(E0  cos  00  -  //?)2  +  (Eg  sin  00  -  IX)2,  (B) 

which  equation  expresses  the  value  of  the  motor  e.m.f.  in  terms  of  the  line  voltage, 
current,  phase  angle,  and  armature  constants. 


160       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

A  second  set  of  these  V  curves  is  obtainable  from  the  current- 
power  curve,,  namely,  those  showing  the  relation  between  power 
factor  and  field  excitation.  These  are  determined  as  follows: 
Assume  any  condition  of  constant  input  and  let  this,  as  before,  be 
30  kw.;  then  refer  to  the  current-power  curves  (Fig.  95)  and  mul- 
tiply the  current  at  any  motor  e.m.f.  by  the  line  voltage.  This 
product  is  the  volt-amperes  input.  A  division  of  the  correspond- 
ing abscissa  value  (which  was  30  kw.  input  in  this  instance) 
by  the  volt-amperes  gives  the  power  factor  at  the  selected  field 
excitation  and  input.  The  curves  obtained  by  using  the  power 
factor  and  the  corresponding  motor  e.m.f.'s  as  ordinates  and 
abscissae,  respectively,  are  those  represented  by  the  broken  lines 
in  Fig.  96.  It  is  to  be  noted  that  these  are  the  converse  in  shape 
of  the  current  V  curves.  The  power  factor  rises  with  field  excita- 
tion up  to  about  unity  at  500  volts,  but  as  the  field  strength  of  the 
motor  is  still  further  augmented  the  power  factor  decreases,  even 
more  rapidly  than  it  rose.  It  is  also  apparent  that  the  power  factor 
of  the  machine  is  improved  by  addition  of  load  to  the  motor. 

Reference  to  these  curves  (Fig.  96)  and  to  the  formula  for  motor 
e.m.f.  (Em)  shows  that  the  smaller  values  of  motor  e.m.f.  exist 
when  <j>g  is  a  lagging  angle,  because  then  sin  <j>0  is  plus.  The  larger 
values  of  the  motor  e.m.f.  exist  when  (f>9  is  a  leading  angle,  sin  <j)g 
then  being  minus,  which  increases  the  second  term  under  the  radi- 
cal and  consequently  Em  is  greater.  Hence,  variation  of  the  motor 
excitation  produces  a  change  in  the  angle  between  line  volts  and 
motor  current.  Low  field  excitations  cause  a  lagging  current  to 
flow  and  high  excitations  draw  a  leading  current.  Thus  a  syn- 
chronous motor  whose  e.m.f.  is  greater  than  that  of  the  line  (motor 
super-excited)  draws  a  leading  current  and  acts  as  if  it  possessed 
electrostatic  capacity. 

The  above  property  of  the  synchronous  motor,  used  as  such  or  as 
a  rotary  converter,  is  frequently  utilized  to  improve  the  power 
factor  of  long-distance  transmission  systems,  because  when  energy 
is  supplied  to  induction  motor  loads  they  are  apt  to  produce  a  very 
low  power  factor,  (p.  192)  necessitating  a  large  lagging  current  to  be 
transmitted.  This  condition  not  only  increases  the  line  drop  but 
also  interferes  seriously  with  alternator  regulation.  The  installa- 
tion of  a  super-excited  synchronous  motor  at  the  receiving  end  of 
a  line  reduces  this  angle  of  lag  by  drawing  a  leading  current. 


SYNCHRONOUS  ALTERNATING-CURRENT  MOTOR.         161 

In  fact,  the  motor  field  can  be  so  adjusted  that  the  phase  displace- 
ment between  line  voltage  and  current  becomes  small  or  even  nil. 

Balancing  Action  of  Synchronous  Motor.  — The  fact  that  a  syn- 
chronous motor  draws  a  leading  current  when  super-excited,  and 
that  the  extent  of  lead  is  increased  with  the  degree  of  super-excita- 
tion, gives  to  this  type  of  polyphase  machine  the  property  of 
restoring  a  balance  to  an  unbalanced  polyphase  circuit.  When 
a  slightly  super-excited  motor  is  connected  to  the  terminals  of  a 
balanced  or  an  unbalanced  polyphase  system,  all  phases  of  the 
motor  armature  draw  a  leading  current.  That  phase  winding, 
however,  which  is  connected  across  the  line  terminals  of  lower 
voltage  draws  a  current  of  greater  lead  than  the  other  windings 
because  its  super-excitation  is  relatively  higher;  hence,  the  com- 
pounding tendency  of  the  leading  current  is  more  marked  in  this 
phase  than  in  the  others  and  the  voltage  thereof  is  increased.  This 
action  tends  to  balance  the  circuit,  not  only  in  voltage  but  in  cur- 
rent as  well,  because  combinations  of  leading  and  lagging  currents 
give  reduced  resultant  currents. 

Hunting  of  Synchronous  Motor.  — A  trouble  which  sometimes 
arises  in  connection  with  synchronous  motors  is  that  of  hunting, 
pumping,  or  phase  swinging.  These  terms  signify  the  periodic 
fluctuations  in  speed  and  armature  current  occurring  under  certain 
conditions.  Such  surgings  may  be  produced  by  several  causes. 
Let  us  suppose  the  typical  motor  to  be  running  at  a  constant  load 
up  to  a  certain  instant,  and  that  then  the  load  is  suddenly  varied,  say 
increased.  A  momentary  retardation  of  the  motor  armature  natu- 
rally results,  and  we  see  from  power  curves  (Fig.  89)  that  the  angle 
between  Em  and  Eg  must  decrease  to  allow  of  increase  in  the  driving 
power  of  the  motor.  The  proper  value  of  armature  current  is  not 
obtained  at  the  instant  that  the  correct  phase  angle  between  Em  and 
Eg  exists,  but  somewhat  later,  owing  to  time  lag  caused  by  inertia 
and  the  inductance  of  the  armature  winding.  Thus,  while  the 
armature  may  momentarily  pass  through  the  right  angular  relation 
with  respect  to  the  line  e.m.f.,  it  will  ultimately  draw  more  current 
than  its  load  requires.  This  causes  the  armature  to  be  accelerated 
and  the  angle  (d)  between  Em  and  Eg  is  increased  beyond  its  proper 
value,  until  the  driving  torque  is  so  much  reduced  as  to  be  insuffi- 
cient for  the  motor  load,  whereupon  lagging  or  retardation  of  the 
armature  again  ensues,  and  so  on.  This  phase  swinging  of  the  arma- 


162       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

ture  and  current  variation  accompanying  are  included  under  the 
term  hunting.  The  amplitude  of  this  swinging  or  pendular  motion 
usually  dies  down,  due  to  frictional,  eddy  current,  and  hysteretic 
effects,  so  that  the  armature  finally  finds  its  correct  load  position. 
The  irregular  rotation  above  considered  may  be  regarded  as  con- 
sisting of  a  uniform  motion  of  rotation  at  synchronous  speed  with 
a  to-and-fro  or  pendular  motion  superposed  upon  it. 

We  see,  therefore,  that  a  change  of  load  upon  a  synchronous  motor 
will  produce  oscillations  in  its  angular  velocity  and  armature  current. 
There  are,  however,  other  actions  by  which  such  fluctuations  may  be 
started.  Assume  the  motor  load  to  remain  quite  constant  but  the 
speed  of  the  generator  to  undergo  a  sudden  rise.  This  corresponds 
to  an  advance  of  the  angular  position  of  the  line  voltage  vector, 
which  naturally  decreases  the  angle  between  Em  and  Eg,  thus  increas- 
ing the  driving  torque  of  the  synchronous  motor  (see  operative- 
range  curves,  Fig.  89),  and  producing  an  acceleration,  which  will, 
as  already  shown,  develop  the  hunting  phenomenon.  Similarly 
a  sudden  change  in  excitation  of  the  motor  or  in  the  line  voltage  will 
produce  a  variation  in  the  motor  torque  and  thus  bring  about  the 
phase  surging  action.  This  fluctuation  is  very  slow  compared  with 
the  line  frequency,  and  its  period  can  be  readily  determined  by  the 
violent  swinging  of  the  needle  of  an  ammeter  connected  in  the 
supply  line. 

As  already  stated,  the  hunting  started  by  any  one  sudden  dis- 
turbance will  gradually  subside.  If,  however,  before  the  oscilla- 
tions due  to  one  cause  have  been  damped  out,  another  disturbance 
should  occur  of  such  nature  as  to  reenforce  the  already  existing 
fluctuations,  their  combined  amplitude  may  become  so  great  as  to 
cause  the  motor  armature  to  swing  beyond  the  range  of  stability, 
with  the  result  that  it  falls  out  of  step  and  stops.  This  action  is 
likely  to  occur,  because  sudden  load  changes  are  usually  accom- 
panied by  marked  variations  in  line  voltage,  both  acting  to 
produce  a  like  effect.  The  surges  of  the  armature  current  thus 
developed  may  affect  the  alternator  speed,  and  then  all  three  dis- 
turbing factors  act  in  unison.  If  the  phase  swinging  is  not  violent 
enough  to  cause  the  motor  to  pull  out  of  step,  the  variation  thus 
produced  in  the  line  voltage  may  be,  nevertheless,  of  such  low 
periodicity  that  it  becomes  apparent  in  the  flickering  of  lamps  con- 
nected to  the  circuit,  and  it  may  even  develop  hunting  in  other 


SYNCHRONOUS   ALTERNATING-CURRENT  MOTOR. 


163 


synchronous  motors  fed  thereby.  The  fact  that  hunting  of  a  syn- 
chronous motor  not  only  interferes  with  its  own  stability  but  may 
react  upon  other  synchronous  units  connected  to  the  line  is  very 
objectionable,  and  the  prevention  of  marked  hunting  becomes  a 
practical  necessity. 

Prevention  of  Hunting.  — Inspection  of  the  power  or  operative 
range  curves  of  synchronous  motors  (Fig.  89)  shows  that  a  given 


FIG.    97. — THE   HUTIN   AND   LE   BLANC   AMORTISSEUR   OR   DAMPER. 


FIG.    98.  —  ANTI-HUNTING   DEVICE. 

change  in  torque  will  be  obtained  with  a  smaller  phase  swing  when 
the  field  is  strong  than  when  it  is  weak,  and  naturally  the  smaller 
the  initial  phase  shift  the  less  the  resulting  oscillation;  hence,  use  of 
strong  fields  is  favorable  in  checking  hunting,  but  this  by  itself  will 
not  suffice. 

It  has  already  been  indicated  that  the  production  of  eddy  currents 
by  the  surging  of  the  armature  current  tends  towards  the  reduction  of 
the  duration  of  the  surges,  and  theoretically  any  device  wherein 
currents  are  generated  by  the  pendular  motion  of  the  armature  will 
act  by  Lenz's  law  to  stop  the  motion  producing  them.  Hence  it  is 


164      ELECTRIC  MOTORS,  THEIR  ACTION  AND  CONTROL. 

necessary  to  design  a  device  wherein  heavy  eddy  currents  are 
developed  by  even  slight  hunting  tendencies,  in  order  to  arrest  the 
surging  in  its  very  development.  The  devices  producing  this 
checking  effect  are  known  as  damping  coils  or  dampers.  One  of  the 
earliest  of  these  dampers  was  proposed  by  Hutin  and  Le  Blanc* 
(Fig.  97),  and  consists  of  a  series  of  thick  copper  bars  embedded  in 
each  pole  piece,  parallel  to  the  armature  axis,  and  connected  in 
parallel  by  heavy  copper  rings  concentric  with  the  armature.  A 
more  modern  and  probably  more  economical  arrangement  (Fig.  98) 
consists  of  a  copper  grid  placed  in  corresponding  slots  cut  in  the 
polar  face,  the  outer  rim  of  the  grid  forming  a  closed  band  around 
the  pole  piece. f 

*  U.  S.  Patent  No.  529,272,  November  13,  1894. 
f  U.  S.  Patent  No.  575,116,  January  12,  1897. 

For  further  information  concerning  synchronous  motors  see  : 

ALTERNATING  CURRENTS,  D.  C.  and  J.  P.  Jackson,  p.  571. 

ALTERNATING  CURRENT  PHENOMENA.    C.  P.  Steinmetz.     1908. 

DIE  WECHSELSTROMTECHNIK,  Vol.  IV.     E.  Arnold.     1904. 

ELECT.  ENG.  POCKET-BOOK,  p.  430. 

LONDON  INST.  Civ.  ENGS.     J.  Hopkinson.     1883. 

STANDARD  HAND-BOOK,  p.  431.    McGraw.     1908. 

SYNCHRONOUS  MOTOR  DIAGRAMS.     A.  S.  McAllister.    Elect.  World,  August,  1907. 

SYNCHRONOUS  MOTORS.     Prof.  C.  A.  Adams.     Harvard  Eng.  Journal,  1907. 

SYNCHRONOUS  MOTORS.    F.  G.  Baum.     Electric  World,  March,  1902. 

Trans.  A.  I.  E.  E.,  Vol.  XIX,  1902,  p.  718.     C.  P.  Steinmetz. 

Trans.  A.  I.  E.  E.,  Vol.  XXIII,  1904,  p.  481.     G.  B.  Lamme. 

Trans.  A.  I.  E.  E.,  Vol.  XXVI,  1907,  p.  1027.    M-  Brooks. 


CHAPTER    XIV. 


POLYPHASE  INDUCTION  MOTORS. 

THE  polyphase  induction  motor  as  developed  through  the  inven- 
tions of  Ferraris,  Tesla,  and  others  is  undoubtedly  the  most  impor- 
tant of  alternating-current  motors.*  Two-phase  or  three-phase 
machines  are  employed,  depending  upon  the  system  by  which  the 
current  is  supplied.  The  operation  of  the  induction  motor  is  very 
different  from  that  of  the  preceding  types  because  there  is  no  elec- 
trical connection  between  the  armature  (usually  called  secondary 
or  rotor)  and  the  source  of  current  supply.  The  motion  of  the  arma- 
ture is  produced  by  a  rotating  magnetic  field,  and  it  is  this  peculiar 
field  which  is  the  characteristic  of  induction  motors. 

Production  of  Rotary  Field.  —  A  laminated  iron  ring,  wound  with 
insulated  wire,  as  represented  in  Fig.  99,  is  supplied  with  two- 
phase  or  quarter-phase  currents 
at  four  equidistant  points  A,  B,  C 
and  D.  Two  conductors  of  one 
phase  are  connected  at  A  and 
B,  and  those  of  the  other  phase 
across  C  and  D  respectively.  The 
direction  of  the  winding  is  such 
that  a  current  entering  at  A  will 
produce  a  south  pole  at  this  par- 
ticular point  and  a  north  pole  at 
B,  therefore  if  a  compass  needle 
were  placed  inside  of  the  ring,  it 
would  tend  to  point  vertically  up- 
ward as  indicated  by  the  dotted 
arrow.  This  condition  is  represented  at  i  in  Fig.  100,  the  current 
of  phase  AB  having  its  maximum  positive  value,  and  that  of  phase 
CD  zero  value,  in  accordance  with  the  usual  phase  difference  of 
90°  or  one-quarter  period  existing  between  two-phase  currents. 
A  moment  later,  i.e.,  one-eighth  of  a  period,  the  current  in  AB 

*  See  pages  125-128. 
165 


FIG.  99.  —  RING  SUPPLIED  WITH 
TWO-PHASE  CURRENTS. 


166      ELECTRIC  MOTORS,    THEIR   ACTION   AND   CONTROL. 

has  decreased  somewhat,  and  the  other  has  increased,  so  that  they 
are  now  equal.  In  this  case,  each  current  will  tend  to  produce  a 
south  pole  where  it  enters  the  winding  at  A  and  D  respectively,  so 
that  a  resultant  polarity  is  developed  midway  between,  as  shown 
at  point  2,  the  arrow  being  inclined  at  an  angle  of  45°.  The  next 
instant,  the  current  of  phase  AB  has  fallen  to  zero,  and  that  of 
CD  has  reached  its  maximum,  so  that  the  needle  takes  the  hor- 
izontal position  as  represented  at  3  in  Fig.  100.  Again  at  135°,  the 
current  AB  has  reversed,  tending  to  make  a  south  pole  at  B, 
the  needle  being  inclined  downward  at  an  angle  of  45°  as  shown 
at  point  4.  By  following  the  successive  conditions,  the  needle  will 


TIG.  100.  —  MAGNETIC  RESULTANTS  DUE  TO  TWO-PHASE  CURRENTS. 

be  found  to  take  the  various  positions  represented  at  points  5,  6, 
7,  8,  and  finally  at  9  it  assumes  its  original  vertical  direction,  the 
current  having  then  completed  one  cycle  of  its  changes,  having 
passed  through  two  alternations.  Thus,  the  compass  needle  tends 
to  be  rotated  on  its  support  continuously  by  the  shifting  result- 
ant field,  as  long  as  the  winding  is  supplied  with  two-phase  or 
quarter-phase  currents.  If  either  one  of  the  connections  AB  or  CD 
(Fig.  99;  were  reversed,  the  direction  of  rotation  of  the  needle  would 
then  be  counter-clockwise,  instead  of  clockwise.  Hence,  to  reverse 
the  direction  of  rotation  of  such  a  field,  it  is  necessary  to  interchange 
the  terminals  of  one  of  the  two  phases. 

The  Action  of  Three-Phase  Currents  in  producing  a  rotary  field 
is  quite  similar  to  that  explained  for    two-phase   currents.     The 


POLYPHASE   INDUCTION   MOTORS. 


167 


laminated  ring  of  Fig.  101  is  wound  as  before,  but  the  current  is 
led  in  at  the  three  equidistant  points  X,  Y  and  Z,  instead  of  at 
four  points,  as  was  indicated  for 
two-phase  currents.  Taking  the 
instant  when  the  current  flowing 
in  at  .Y  is  a  maximum,  then  cur- 
rents flowing  out  at  Y  and  Z  each 
have  one-half  the  value  of  that 
entering  at  X.  This  tends  to 
produce  a  south  pole  at  X,  and 
two  north  poles  at  Z  and  Y 
respectively.  The  resultant  due 
to  the  latter  is  a  north  pole  at  T, 
midway  between  Y  and  Z;  conse- 
quently a  magnetic  needle  placed 
within  the  ring  would  assume  the  position  indicated  by  the  dotted 
arrow  at  i  in  Fig.  102.  One-sixth  of  a  period  later,  currents 
enter  at  both  X  and  Z,  and  a  maximum  current  flows  out  at  Y, 


FIG.  101.  —  RING  SUPPLIED  WITH 
THREE-PHASE  CURRENTS. 


FIG.  102.  —  MAGNETIC  RESULTANTS  DUE  TO  THREE-PHASE  CURRENTS. 

hence  the  needle  would  point  towards  V.  At  the  end  of  another 
one-sixth  of  a  period,  the  maximum  current  would  enter  at  Z,  and 
the  needle  would  turn  to  that  point  as  shown  at  3  in  Fig.  102, 
and  so  on  until  it  had  made  a  complete  revolution  in  one  period 
of  the  alternating  current.  If  any  of  the  two  connections  shown 


168      ELECTRIC  MOTORS,    THEIR   ACTION  AND   CONTROL. 

in  Fig.  101  be  transposed,  the  direction  of  rotation  of  the  corre- 
sponding magnetic  field  will  be  reversed. 

Variation  of  Flux  with  Two-Phase  and  Three-Phase  Stator  Wind- 
ings. —  If  the  stator  is  wound  for  two-phase  currents  as  in  Fig.  100 
we  have  an  induction  or  flux  produced  which  varies  between  two 
limits,  namely  between  that  developed  when  the  current  in  phase 
AB  is  a  maximum,  and  that  existing  when  the  currents  in  the  two 
phases  are  equal.  Numerically  this  variation  is  between  ^/raax 
and  2  jfiT/max  sin  45°,  or  proportionally  between  i  and  1.414;  that 
is,  there  is  variation  of  about  20  per  cent  from  its  mean  value. 
This  variation,  however,  becomes  considerably  reduced  by  the 
effects  of  the  rotor  currents  and  the  storage  of  electro-magnetic 
energy. 

The  currents  in  the  case  of  the  three-phase  winding  (Fig.  101) 
may  be  taken  as  /max  sin  cot,  7max  sin  (a)t  +  120)  and  7max  sin  (tot  + 
240)  being  represented  in  Fig.  102.  The  maximum  flux  is  produced 
at  the  instants  corresponding  to  positions  i,  2,  3,  4,  etc.,  in  Fig.  102, 
and  the  minimum  flux  is  developed  at  instants  midway  in  time 
between  those  shown. 

The  maximum  value  of  the  flux  may  be  written 

^max  -  #/max  +  2  #/max  Sin  3O°  =   2  #/max 

and  the  minimum  value  is 

Jkfmax  =  0+2  #/max  sin  6o°  =   1-73  #/max. 

Thus  the  flux  varies  between  limits  which  are  proportional  to  2  and 
1.73,  or  the  variation  from  its  mean  value  is  approximately  7  per 
cent. 

The  above  comparison  would  indicate  that  an  increase  in  the 
number  of  phases  employed  to  produce  the  flux  in  the  stator  of  an 
induction  motor  would  dimmish  the  per  cent  variation  from  its 
mean  value. 

The  smooth  running  of  a  motor  depends  to  a  certain  extent  upon 
the  uniformity  of  the  turning  effort  exerted  upon  the  rotor,  and  con- 
sequently upon  the  regularity  of  the  magnetic  field  in  which  it 
revolves.  It  might  accordingly  be  supposed  that  the  greater  the 
number  of  phases  for  which  the  stator  is  wound,  the  steadier  would 
be  the  action  of  the  motor.  In  practice,  however,  this  does  not 
obtain,  no  noticeable  difference  in  smoothness  of  rotation  being 
found  between  two-phase  and  three-phase  machines. 


POLYPHASE  INDUCTION  MOTORS.  169 

The  particular  advantage  of  the  three-phase  motor  with  respect  to 
the  two-phase  machine  is  that  it  is  more  economical  as  regards 
copper  for  its  stator  winding,  since  the  smaller  current  per  phase 
of  the  former  would  produce  an  equivalent  induction.  The  three- 
phase  winding  also  lends  itself  better  to  the  production  of  a  simple 
starting  device,  being  connected  in  "Y"  at  starting  and  in  delta 
for  running.  In  practice  three-phase  machines  are  more  gener^ 
ally  employed,  because  the  corresponding  generators,  transformers, 
transmission  lines,  etc.,  are  more  economical  of  material. 

The  ring  with  the  magnetic  needle  just  described  illustrates  the 
synchronous  polyphase  motor,  since  the  armature  revolves  in  exact 
synchronism  with  the  phases  of  the  currents.  If  the  needle  is 
replaced  by  a  cylinder  of  laminated  iron  wound  with  inductors 
like  an  ordinary  armature  except  that  they  are  short-circuited,  it  is 
found  that  this  will  also  revolve,  but  in  this  case  the  speed  is  a  little 
less  than  that  of  the  synchronous  armature.  The  difference  in 
speed  (angular  velocity)  between  the  rotary  field  and  the  armature 
divided  by  that  of  the  former  is  called  the  slip;  or  denoting  the  slip 
by  S,  the  angular  velocity  of  the  rotary  field  by  wx  and  that  of  the 

armature  by  o>2,  we  have: 

0      co.  —  a>~  f     . 

S  =  -1 2  •  (24) 

*>i 

This  slip  represents  a  relative  motion  of  the  rotating  field,  with 
respect  to  the  armature  inductors;  consequently  the  latter  are  cut 
by  lines  of  force  and  therefore  currents  are  induced  in  them.  Since 
it  is  the  action  of  the  field  upon  these  induced  currents  which  causes 
the  armature  to  revolve,  this  type  of  machine  is  called  the  induction 
motor.  It  is  to  be  noted  that  no  current  is  supplied  to  the  moving 
part,  hence  it  need  have  no  electrical  connections  made  to  it,  except 
(as  will  be  shown  later)  for  purposes  of  starting  and  regulation,  in 
which  case  electrical  connection  is  necessary. 

The  stationary  part  of  the  typical  motor  is  that  connected  to  the 
source  of  current  supply,  and  it  is  usually  termed  the  stator  or 
primary.  The  moving  part  is  called  the  rotor  and  forms  a  secondary 
to  the  stator. 

The  terms  field  and  armature  could  without  error  be  retained, 
because  the  primary  forms  the  inducing  member  or  field,  while  the 
secondary  or  rotor  is  that  part  acted  upon  inductively,  or  the  arma- 
ture. 


170      ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

Typical  Induction  Motor.  —  The  type  of  winding  illustrated  in 
the  development  of  the  rotary  field  does  not  lend  itself  to  the  pro- 
duction of  a  commercial  machine  on  account  of  the  waste  of  copper 
and  its  high  leakage  reactance.*  The  rotor  winding  and  core  must 
also  be  modified  to  suit  practical  conditions. 

The  typical  stator  core  consists  of  an  assemblage  of  thin  iron  or 
mild  steel  rings  of  about  .014  to  .025  inches  in  thickness,  with  teeth 
and  slots  upon  the  inner  circumference.  These  slots  contain  a 
distributed  drum  winding  of  substantially  the  same  character  as  the 
armature  winding  of  polyphase  alternators.  The  magnetic  poles 
are  therefore  not  produced  by  windings  concentrated  at  certain 
points  of  the  gap  periphery  on  salient  or  separately  projecting 
masses  of  iron  as  in  d.  c.  machines.  Nevertheless,  magnetic  poles 
are  formed  by  properly  connecting  the  groups  of  coils.  Although 
a  diagram  as  in  Fig.  99  may  be  used  to  represent  the  stator  winding 
for  theoretical  discussion,  it  does  not  portray  the  actual  commercial 
machine.  The  windings  are  seldom  closed-coils,  the  three-phase 
stator  is  usually  Y  connected,  although  certain  manufacturers 
employ  this  grouping  simply  for  starting,  changing  to  delta  con- 
nection when  running. 

The  winding  is  divided  into  a  number  of  groups,  equal  to  the 
product  of  the  number  of  phases  and  the  number  of  poles.  Fig.  103 
represents  the  diagram  of  an  8-pole  two-phase  winding.  Consider 
the  instant  when  the  currents  in  the  two  phases  are  in  the  same 
direction  (that  is  between  o°  and  90°  or  180°  and  270°,  Fig.  TOO), 
then  by  tracing  out  the  connections,  it  will  be  found  that  the  currents 
circulate  in  the  same  direction  in  two  adjacent  groups.  Thus  a 
pole  is  formed  by  two  groups,  both  phases  being  represented  in  each 
pole.  When  the  current  in  each  phase  reverses  (after  a  half  cycle) 
the  pole  shifts  the  angular  distance  covered  by  two  groups,  so  that 
the  field  completes  one  revolution  in  eight  alternations  of  current. 
Thus  if  the  current  supplied  had  a  frequency  of  60  cycles  per 
second,  the  field  would  make  15  revolutions  per  second,  or  900  per 
minute. 

To  minimize  the  length  of  cross-connecting  wire,  it  will  be  seen 
that  every  fourth  group  is  connected  in  the  same  direction  in  each 
phase.  A  coiled  arc  such  as  A  represents  a  group  comprising  a 

*  Leakage  reactance  is  that  component  of  the  inductive  reactance,  due  to  such 
lines  of  force  (stray)  as  are  not  effective  in  the  production  of  torque. 


POLYPHASE  INDUCTION  MOTORS. 


171 


certain  number  of  coils  in  series,  each  coil  being  located  in  a  sepa- 
rate pair  of  slots  and  the  end  of  one  being  connected  to  the  beginning 
of  the  next. 


FIG.  103.  —  EIGHT-POLE   TWO-PHASE   STATOR   WINDING. 

A  six-pole  three-phase  winding  of  18  groups  is  indicated  in  Fig.  104. 
The  phases  are  represented  in  counter-clockwise  direction  in  the 
order  A,  B,  C,  A',  B',  C',  analogous  to  the  two-phase  winding. 
The  phases  are  thus  only  60  degrees  apart.  To  get  the  star  or  Y, 
which  is  a  i2o-degree  relation,  the  middle  phase  is  reversed,  as  in 
Fig.  105,  so  that  a  pole  will  be  formed  by  the  three  consecutive 
phases  when  the  current  is  in  the  same  direction  in  A  and  C,  and 
opposite  in  B.  The  beginning  of  the  middle  coil  (C),  and  not  the 
end,  as  with  the  other  two,  is  connected  to  the  common  point  O. 


B       C      A 


FIG.  104.  —  SIX-POLE  THREE-PHASE  STATOR  WINDING. 


172 


FIG.   105.  —  REVERSAL  OF  MIDDLE  COIL  WITH   6o°  SPACING  TO  OBTAIN 
120°    STAR   ARRANGEMENT. 


POLYPHASE  INDUCTION  MOTORS.  173 

In  this  case  the  pole  shifts  the  distance  of  three  groups  for  each 
alternation,  so  that  one  revolution  of  the  field  is  completed  in 
three  periods,  making  20  r.p.s.  or  1200  r.p.m.  with  60  cycle 
current. 

The  speed  or  number  of  revolutions  made  by  the  rotating  field 
accordingly  depends  upon  the  frequency  as  well  as  upon  the  num- 
ber of  poles,  being  directly  as  the  former  and  inversely  as  the  latter,  or 

r.p.m.  =  60  X  frequency  -r-  pairs  of  poles.  (25) 

Since  speeds  of  more  than  1800  r.p.m.  are  higher  than  can  con- 
veniently be  employed,  the  majority  of  induction  motors  have  four, 
or  a  still  greater  even  number  of  poles.  For  example,  a  group 
of  commercial  60  cycle  machines  has  two  pairs  of  poles  up  to  5  horse- 
power capacity,  three  pairs  from  7.5  to  30  horsepower,  four  pairs 
from  30  to  50  horsepower  and  five  or  six  pairs  for  sizes  between 
50  and  200  horsepower. 

The  rotor  core  consists  of  a  laminated  iron  cylinder,  with  the 
winding  either  of  copper  bars  or  of  wires  embedded  in  it. 

The  simplest  form  of  rotor  construction  employs  what  is  known 
as  the  squirrel-cage  winding,  which  was  devised  by  Dobrowolsky. 
It  consists  of  a  number  of  lightly  insulated  copper  rods  or  bars 
arranged  in  holes  or  slots  around  the  rotor  periphery,  and  con- 
nected at  each  end  by  brass  or  copper  rings  of  ample  cross-section. 
There  must  be  no  common  factor  between  the  number  of  rotor 
and  stator  slots,  otherwise  the  latter  may  tend  to  "lock,"  or  fail 
to  start  when  current  is  supplied  to  the  stator  winding.  The  end 
rings  may  be  solid  or  laminated  copper  punchings,  and  connection 
to  the  bars  can  be  made  by  means  of  rivets,  screws  or  solder. 
Riveting  is  expensive  in  labor,  and  if  not  done  well  gives  poor 
contact,  which  results  in  heating  and  large  slip.  Screws  and  bolts 
are  also  expensive  and  poor  contacts  are  likely  to  exist.  Soldering 
by  itself  secures  good  contact  but  at  heavy  overloads  or  slow  start- 
ing it  is  likely  to  melt.  Hence  a  combination  of  screws  or  rivets 
with  soldering  is  usually  employed.  In  the  example  of  squirrel- 
cage  winding  which  Fig.  106  illustrates,  the  rotor  has  a  number 
of  equidistant  rectangular  holes  near  its  periphery,  and  through 
these  holes  pass  copper  rods,  the  projecting  ends  of  which  are 
bolted  and  soldered  to  two  cast  metal  end  rings.  A  type  of  rotor 
winding  frequently  adopted  is  similar  in  form  to  the  three-phase 


174      ELECTRIC  MOTORS,    THEIR   ACTION   AND    CONTROL. 

Y  stator  winding  already  described,  but  the  three  free  ends  of  the 
winding  are   led   to   three  slip  rings  upon   which   brushes    bear 


FIG.  106. SQUIRREL-CAGE  ROTOR. 

(Fig.  107).     These  brushes  are  connected  by  leads  to  a  variable 
resistance,  the  function  of  which  will  be  considered  later. 


FIG.  107.  —  SLIP-RING   (WOUND)   ROTOR. 

Fundamental  Equation  of  the  Induction  Motor.  — The  funda- 
mental equation  of  the  induction  motor  is  the  same  as  that  of  the 
transformer,  with  the  exception  of  the  winding  constant  Kr 

•E  =  KI  V27rf  N  3>m  icT8,  (26) 

where  E  =  the  c.e.m.f.  in  volts,  N  the  turns  in  series  per  phase, 
/  =  cycles  per  second  and  $max  the  total  maximum  flux  per  pole. 
K^  is  a  constant  required  to  correct  for  the  departure  of  the  flux 
distribution  from  the  true  sine  wave  form,  and  its  value  varies 
between  .35  and  .95,  depending  upon  the  number  of  phases  and 
inductors  per  stator  slot. 


POLYPHASE   INDUCTION   MOTORS.  175 

The  total  average  induction  around  the  air  gap  then  becomes 

,  where  P  —  number  of  poles. 


V2  2KlNf 

120  4.25  E  io9 


Putting   P$a  =  &  and   -  =  r'P'm'  we  have, 

P         120 


2V2KlNr.p.m.      K,N  T.p.m. 

Formulae  26  and  27  show  that  for  a  given  induction  or  flux  the 
turns  of  winding  are  directly  proportional  to  the  voltage  and  inversely 
as  the  speed.  With  a  given  motor  winding  the  flux  varies  directly 
as  the  volts  and  inversely  as  the  frequency. 

An  equation  for  the  magnetizing  current  of  the  induction  motor 
may  be  developed  as  follows: 

To  produce  a  certain  flux  density  in  the  gap,  a  number  of  ampere 
turns  /  •  Na  are  required  for  the  path  through  air,  and  a  number 
/  •  Ni  are  necessary  for  the  path  through  the  iron.  Then  the 
magnetization  factor,  or  the  total  m.m.f.  in  terms  of  that  required 
to  produce  the  necessary  flux  in  the  air  path,  is 

+i.  (28) 


This  quantity  varies  in  actual  design  from  i.i  to  1.5.  That  is, 
the  ampere-turns  required  for  the  gap  are  the  controlling  factor. 
The  /  •  Ni  may  be  calculated  from  magnetization  curves  of  the 
punchings  employed,  and  the  /  •  Na  may  be  obtained  from  the 
relation 

LNa  =  .3133  $mL0  +  S.  (29) 

Where  S  the  total  surface  of  air  gap  is  the  length  of  core  X  cir- 
cumference, Lg  is  the  effective  length  of  mechanical  gap  X  2.  All 
values  of  the  above  are  expressed  in  inches.  The  magnetizing 
current  7mag  corresponding  to  the  air  gap  should  be  as  low  as  possi- 
ble because  it  reduces  the  power  factor  of  the  motor.  It  can  readily 
be  obtained  with  the  magnetization  factor  evaluated,  being 

/mag  -7«-  X  M.F.  (30) 

The  value  of  7mag  varies  from  15-30  per  cent  of  the  rated  load 
current,  depending  upon  the  size  of  the  motor,  the  larger  per  cent 
being  for  the  smaller  sizes. 


176       ELECTRIC   MOTORS,    THEIR   ACTION   AND   CONTROL. 
The  value  of  Ia  is  given  by  the  equation 


The  magnetization  volt-amperes  will  therefore  be 


m0 

ViNS  (3  ; 

Combining  this  with  equation  28,  we  get  for  a  given  motor  the 
relation 

2.F.  -        ,    (33) 


That  is,  the  magnetizing  volt-amperes  for  a  certain  magnetic 
circuit  are  proportional  to  the  square  of  the  voltage  and  to  the 
square  of  the  number  of  poles,  while  inversely  proportional  to  the 
square  of  the  number  of  turns  and  to  the  frequency.  It  is  evident 
from  this  that  to  keep  the  same  percentage  of  magnetizing  current, 
the  turns  and  volts  must  be  proportional  if  one  or  the  other  change. 
Similarly  with  a  change  in  the  frequency,  the  volts  should  vary  as 
the  square  root  of  the  frequency. 

The  leakage  reactance,  or  those  portions  of  primary  and  second- 
ary reactances  which  are  due  to  leakage  of  flux,  is  difficult  to 
determine  accurately  without  tests.  It  may  be  predetermined 
within  about  ten  per  cent  by  means  of  such  a  formula  as  given  by 
Professor  C.  A.  Adams  (A.  I.  E.  E.  Transactions,  June,  1905).  It 
may  be  expressed  in  reactance  ohms  or  inductive  volts  per  ampere, 
or  per  cent  of  total  flux.  There  are  four  components  comprising 
the  total  leakage,  namely,  primary,  secondary,  zig-zag,  and  end- 
leakage.  Each  of  these  four  factors  is  proportional  to  the  ampere 
turns  per  slot.  The  slot  leakage,  primary  and  secondary,  varies 
inversely  as  the  slot-width,  and  directly  as  the  slot  depth,  the  exact 
functions  being  quite  complex.  The  zig-zag  leakage,  threading 
from  primary  to  secondary  slots,  varies  inversely  as  the  air  gap  length. 
The  end  leakage  varies  roughly  with  the  throw  or  circular  span  of 
the  coils,  or  inversely  as  the  number  of  poles.  A  certain  number 
of  corollaries  follow  from  the  above  relations. 

(a)  Either  of  the  quantities  which  determine  a  low-speed  motor, 
i.e.,  low  frequency  or  large  number  of  poles,  increases  the  per  cent 


POLYPHASE   INDUCTION   MOTORS.  177 

of  leakage  for  a  given  total  induction  by  decreasing  the  flux  per 
pole. 

(b)  The  per  cent  leakage  varies  inversely  as  the  square  of  the 
voltage,  since  for  a  given  apparent  watts  input,  the  current  and 
the  flux  per  pole  respectively  vary  inversely  and  directly  as  the 
voltage. 

(c)  The  effect  of  the  slot  openings  is  to  cut  down  the  slot  leakage 
flux.     Hence  the  use  of  open  secondary  slots,  even  where  the  con- 
ductors are  placed  in  slots  from  the  ends  and  not  from  above  as  in 
the  primary. 

The  leakage  current  is  that  additional  magnetizing  current  re- 
quired to  maintain  the  primary  flux  against  the  secondary  reactions. 
It  may  be  determined  from  tests,  very  easily  and  with  considerable 
accuracy;  either  from  pull  out  (or  maximum  torque),  or  from  the 
locked  current  (which  is  the  current  drawn  by  the  motor  when 
rated  voltage  is  applied  to  the  stator  with  the  rotor  held  stationary). 
The  following  empirical  relation  between  pull  out  torque  and  per 
cent  leakage  has  been  found  to  hold : 

_.  ,11  40  Rated  load  torque  ,     x 

Per  cent  leakage  =  — —• — 3 (34) 

Pull  out  torque 

If  readings  of  voltage,  amperes  and  watts  are  taken  with  the  rotor 
locked,  the  leakage  ohms  (toi)  are: 


(35) 

However,  since  the  conditions  are  even  more  exaggerated  than 
they  are  in  a  transformer  with  its  secondary  short-circuited,  the 
mutual  flux  is  reduced  to  the  very  small  value  required  to  maintain 
the  current  through  such  an  exceedingly  low  resistance,  and  the 
magnetizing  current  is  relatively  also  very  low.  Under  these  con- 
ditions the  percentage  leakage  will  be  given  by: 

Per  cent  leakage  =      ^       •  (36) 

Where  /  equals  rated  load  current,  cos  <j>  equals  full  load  power 
factor,  E  equals  rated  voltage,  and  Irl  equals  rated  load  primary 
drop.  The  denominator  only  approximates  the  useful  or  c.e.m.f., 
since  it  does  not  take  into  account  the  Ix  drop;  but  will  be  found 


178       ELECTRIC  MOTORS,    THEIR  ACTION   AND   CONTROL. 


satisfactory  as  far  as  practical  results  go,  although  not  rigorously 
exact. 

The  power  factor  of  an  induction  motor  may  be  determined  for 
any  load,  from  the  per  cent  value  of  the  two  wattless  components 
of  the  current  input,  by  means  of  the  relation : 

cos  (/>  =  Vioo2  —  (per  cent  magnetization  +  per  cent  leakage)2 

=  Vioo2  -  (M  +  LY  .  (37) 

wherein  M  and  L  are  the  respective  values  of  the  magnetization  and 
leakage  currents  in  per  cent  of  the  total  current. 

The  magnetization  current  is  substantially  the  same  at  all  loads, 
hence  its  percentage  varies  inversely  with  the  load.  The  leakage 
current,  however,  is  a  direct  function  of  the  load,  being  substantially 
zero  at  no  load. 

To  show  the  effect  of  various  relative  values  of  percentage  leakage 
and  magnetization  currents  the  following  example  is  given,  the 
selected  motors  having  the  same  value  of  M  +  L  at  rated  load. 

EFFECT  OF  LEAKAGE  AND  MAGNETIZATION  CURRENTS  UPON 
MOTOR  POWER  FACTOR. 


Load. 

Motor  No.  1. 

Motor  No.  2. 

Motor  No.  3. 

50% 
Rated 

Per  cent  L 
Per  cent  M 
P.F. 

5 
60 
76 

10 
40 
86.6 

15 
20 
93.5 

Rated 

Per  cent  L 
Per  cent  M 
P.F. 
Pull  Out  Torque* 

10 
30 
91.7 
4 

20 
20 
91.7 
2 

30 
10 
91.7 
1.33 

125% 
Rated 

Per  cent  L 
Per  cent  M 
P.F. 

12.5 
24.0 
93 

25 
16 
91.3 

37.5 

8 
89.2 

150% 
Rated 

Per  cent  L 
Per  cent  M 
P.F. 

15.0 
20.0 
93.6 

30 
13.3 
90.1 

45.0 
6.6 
85.6 

175% 
Rated 

Per  cent  L 
Per  cent  M 
P.F. 

17.5 
17.2 
93.8 

35.0 
11.4 

88.5 

52.5 
5.7 
81.3 

200% 
Rated 

Per  cent  L 
Per  cent  M 
P.F. 

20 
153.6 

40.0 
86.5 

Pull  Out. 

*  In  terms  of  rated  load  torque. 


POLYPHASE   INDUCTION   MOTORS. 


179 


Examination  of  this  table  indicates  that  motor  No.  i  is  best 
suited  to  heavy  overloads  on  account  of  the  small  percentage  of  its 
leakage  current.  Motor  No.  3  is  best  suited  to  light  loads  by  reason 
of  the  small  percentage  of  its  magnetization  current.  The  curves 
given  in  Fig.  108  are  drawn  from  the  data  of  the  above  table,  per 
cent  load  and  per  cent  power-factor  being  employed  as  abscissae 
and  ordinates  respectively. 


100 


90 


80 


70 


60 


25 


50 


75  100  125 

Percent  Rated  Load 


150 


175 


200 


FIG.   I08. EFFECT  OF  LEAKAGE  AND  MAGNETIZATION  CURRENTS 

UPON  POWER  FACTOR  OF  MOTOR. 

Torque  and  Speed.  — It  was  shown  in  the  development  of  the 
elementary  induction  motor  that  the  phenomenon  which  caused 
the  secondary  of  the  motor  to  revolve  was  the  mutual  action  of  the 
rotary  field  and  the  secondary  currents.  That  is,  the  rotating 
magnetic  field  induces  currents  in  the  secondary,  and  these  currents 
acting  according  to  Lenz's  law  tend  to  stop  the  motion  producing 
them.  The  rotation  of  the  rotary  field  cannot,  however,  be  halted 
by  the  secondary  currents,  but  its  speed  can  be  relatively  reduced; 
that  is,  the  rotor  can  follow  the  field. 

At  no  load  the  e.m.f.  induced  in  the  rotor  need  only  be  extremely 
small,  hence  the  rate  of  relative  motion  between  field  and  secondary 
is  very  low,  and  the  rotor  revolves  at  approximately  synchronous 
speed.  As  the  load  gradually  increases,  the  current  required  in 
the  secondary  becomes  larger;  at  the  same  time  the  frequency  of 
the  secondary  e.m.f.  is  higher.  The  current  does  not  increase  at  the 


180       ELECTRIC  MOTORS,    THEIR   ACTION  AND   CONTROL. 

same  rate  as  the  e.m.f.,  since  the  reactance  of  the  circuit  is  greater, 
hence  the  speed  must  fall  off  more  rapidly  than  the  torque  growth 
would  indicate.  In  addition  to  this,  the  decrease  in  speed  with 
increase  in  torque  is  still  further  accentuated  because  the  second- 
ary current  lag  becomes  great,  consequently  a  proportionately 
larger  current  is  required  to  produce  the  corresponding  torque. 
Finally  magnetic  leakage  becomes  pronounced,  the  effective  flux 
reduced  and  the  speed  must  drop  off  an  extra  amount  to  compen- 
sate for  this  condition.  Ultimately  the  required  rate  of  flux  cutting 
can  no  longer  be  maintained,  and  the  motor  stops  or  becomes 
stalled.  The  exact  form  of  the  speed  torque  curve  depends  upon 
the  relation  existing  between  the  resistance  and  reactance  of  the 
secondary  winding,  and  upon  the  leakage  factor. 

The  general  form  of  the  speed-torque  or  more  correctly  the  torque- 
slip  curve  of  an  induction  motor  is  indicated  by  the  following  equa- 

tion  :  T  =  N2  e2  r2s  -*•  ^  (r2  +  s2  x2)  (38)  * 

wherein  N2  is  the  number  of  turns  per  secondary  circuit. 

e    =  induced  volts  per  turn  at  standstill. 

r2  =  resistance  per  secondary  circuit. 

x2  =  reactance  per  secondary  circuit  at  standstill. 

5    =  rotor  slip. 

0)^  =  angular  velocity  of  the  rotary  field. 

*  The  derivation  of  equa.  38  is  based  upon  relations  existing  between  the  corre- 
sponding quantities  in  a  transformer  as  follows: 

Let  E  be  the  line  voltage  per  primary  circuit,  and  with  the  usual  low  resistance  of 
the  stator  winding,  it  may  be  placed  equal  to  the  voltage  induced  per  primary  circuit 
by  the  rotary  field,  or  if  e  is  the  voltage  induced  per  turn, 

N,e  =  Er 

Similarly  at  standstill  the  secondary  induced  voltage  per  circuit  may  be  written 
E2  =  N2e,  and  at  any  slip  s,  this  secondary  voltage  becomes  SN2e.  This  voltage  has 
two  components,  its  resistance  and  reactance  drops,  or 

SN2e  =  I2Vr*+<?x22  («) 

from  which, 

•  <*) 


The  energy  component  of  the  secondary  current  is  consequently 

SNe  r 


.  .  ,  (c) 

Vr*  +  j»s/   Vr*  +  aV       >   +  * 

This  energy  current,  in  terms  of  the  primary  current,  when  multiplied  by  the  pri- 
mary voltage  corresponds  to  that  part  of  the  motor  input  which  represents  the  power  of 


POLYPHASE  INDUCTION  MOTORS.  181 

An  examination  of  this  torque  formula  indicates  many  of  the 
characteristics  of  the  induction  motor,  for  example: 

i.  The  torque  becomes  a  maximum  when  r2  =  sx2]  this  follows 
directly  by  placing  the  differential  of  equation  (g)  with  respect  to  r 
equal  to  zero: 

fyAr.V     \       ^(r  ?  +  f 


which  placed  equal  to  zero  and  simplified  gives 


r22  +  s22x.2  —  2  r22  =  o 


or  5#2  =  r2  which  is  the  condition  of  maximum  torque  above  stated. 
2.    The  torque  of  an  induction  motor  at  standstill  is 


which   evidently  is  greater  the  less  the  resistance  of  the  motor 
winding,  and  the  lower  the  angular  velocity  of  the  rotary  field. 

3.  The  maximum  torque  of  a  motor  occurring  when  r2  =  sx2 
shows  that  maximum  torque  is  exerted  at  standstill  when  r2  =  x2 
because  5  is  then  unity,  or, 

TV  2,,2 

,  ,. 

(40) 


which  varies  inversely  as  the  resistance  and  consequently  to  produce 
a  great  starting  torque  not  only  should  r2  and  x2  be  equal  but  they 
should  both  be  as  small  as  possible. 

the  rotor.  The  relation  between  these  two  currents  is,  however,  expressed  by  the 
inverse  ratio  of  turns,  or  this  energy  component  of  the  primary  current  is: 

2 


and  when  multiplied  by  the  primary  voltage  El  =  N^  it  gives  the  watts  input  repre- 
senting the  power  of  the  rotor,  or 

r-SlV.V 

Rotor  power  =   ^  +  ^  •  (<?) 

This  quantity,  however,  includes  the  copper  losses  occurring  in  the  rotor,  and  these 

r  S2N  *# 

are  from  equation  (&)  expressed  by  the  term  I^R^  =    22      J    2  ;  thus  the  available  power 

r2  -\-  rx2 

of  the  rotor  becomes: 

*- 


The  torque  exerted  by  the  induction  motor  is  obtained  by  dividing  the  rotor  power  by 
the  rotor  slip  w2  =  Wj  (i  —  5),  or  we  have 

r-jAT.V 
Torque  per  rotor  circuit 


182       ELECTRIC  MOTORS,    THEIR   ACTION   AND    CONTROL. 

4.  The  power  factor  of  the  secondary  circuit  is  expressed  by  the 

relation  2     -  ;  but  since  at  maximum  torque  r2  =  sx2  we 

v  r2  +  s2x2 

have  the  condition  that  the  power  factor  of  the  secondary  at  maxi- 
mum starting  torque  should  be  — -* —  =  0.707. 

v  2  r2 

5.  The  value  of  the  secondary  copper  loss  is  from  equa.  b  (p.  180) 
/2V2  =  r2s2N2e2  -*•  (r2  +  s2x2), which  may  be  written  from  equa.  g 
(p.  181)  as: 

I22r2=  s  torque  wt; 

hence  we  see  that  for  a  given  torque  and  frequency  the  rotor  copper 
losses  vary  directly  as  the  slip.  For  example:  Consider  a  motor 
with  85  per  cent  efficiency  at  rated  load  and  a  slip  of  5  per  cent. 
The  efficiency  with  10  per  cent  slip  at  rated  load  would  be  approx- 
imately 80  per  cent,  and  with  15  per  cent  slip  it  would  be  about 
75  per  cent,  one  per  cent  in  efficiency  being  lost  with  each  per  cent 
increase  in  slip. 

6.  The  input  into  the  motor,  not  considering  the  core  losses, 
primary  copper  losses  or  windage  and  friction,  is  from  equa.  e 
(p.  181): 

sN  2e2r 
Motor  input  =  — — *  2  20  while  from  equa.  /,  same  page,  the  motor 

r2  +  s  x2 

output  is 

Motor  output  =  ^ — 2 — **-    — -• 


Since  the  output  divided  by  input  gives  the  motor  efficiency,  it  is 
apparent  that  per  cent  of  electrical  efficiency  is  equal  to  percentage 
of  synchronous  speed  (i  —  s)  attained  by  the  rotor.  The  total 
losses  of  the  motor  were  not  included  in  the  input  as  above  con- 
sidered, so  that  the  true  motor  efficiency  at  any  load  can  never  equal 
the  percentage  of  synchronous  speed  attained  by  the  rotor. 

7.  Further  inspection  of  equa.  38  (p.  180)  indicates  that  for  a 
given  slip  the  motor  torque  varies  as  e2,  but  the  line  voltage  being 
N^,  it  follows  that  for  a  given  slip,  the  motor  torque  varies  directly 
as  the  square  of  the  line  voltage,  and  conversely  at  a  given  torque  the 
rotor  slip  varies  inversely  as  the  square  of  the  line  voltage. 

Circle  Diagram  of  the  Induction  Motor.  — The  characteristic 
curves  show  how  the  power-factor,  torque,  speed,  efficiency  and 


POLYPHASE   INDUCTION  MOTORS.  183 

current  vary  with  load,  that  is,  they  give  the  performance  of  the 
motor.  The  simplest  method  of  determining  the  series  of  curves 
relating  to  the  induction  motor  is  by  means  of  the  circle  diagram. 
Many  diagrams  of  this  kind  have  been  suggested  since  the  first  one 
was  developed  by  Heyland  and  described  by  him  in  the  "Elektro- 
technische  Zeitschrift "  of  October  n,  1894.  The  majority  of  these 
later  diagrams  are  modifications  which  merely  simplify  construc- 
tion and  the  interpretation  of  results.  The  circle  diagram  is  en- 
tirely based  upon  the  fact  that  the  induction  motor  is  substantially 
a  transformer  with  considerably  increased  magnetic  leakage.  The 
essential  difference  in  action  is  the  fact  that  the  energy  of  the  trans- 
former secondary  appears  in  electrical  form,  whereas  that  of  the  motor 
is  given  out  in  mechanical  form.  The  motor  problem  may  accord- 
ingly be  studied  from  the  transformer  standpoint  by  substituting 
the  equivalent  transformer  shown  in  Fig.  109,  which  replaces  each 

L2 


0000 


FIG.  109. TRANSFORMER   DIAGRAM   OF   THE   INDUCTION   MOTOR. 

phase  of  its  winding.  Let  L1  and  L2  denote  those  portions  of  the 
primary  and  secondary  self-inductance  which  are  due  to  the  leakages 
of  flux  which  contribute  nothing  towards  the  development  of  torque. 
The  non-inductive  resistances  h  and  r2  are  introduced  as  shunts 
in  the  primary  and  secondary  circuits  respectively.  Resistance  h  is 
intended  to  represent  a  loss  proportional  to  the  hysteresis  and  eddy 
currents  of  the  primary  core.  This  loss  is  supposed  to  remain 
practically  constant;  while  this  is  not  exactly  true,  the  fact  that  the 
primary  copper  loss  increases  as  the  load  is  augmented,  largely 
compensates  for  the  error  of  this  assumption. 

Let  /j  be  the  primary  current,  r^  the  true  primary  resistance  and 
E  the  primary  voltage  per  phase.  This  latter  may  be  considered 
as  being  made  up  of  three  components,  namely :  * 

*  The  transformer  method  here  employed  was  originally  proposed  by  C.  F.  Bedell 
and  the  development  of  current  locus  is  substantially  that  given  by  J.  Bethenod, 
L'Eclairage  Electrique,  Vol.  XL,  page  253,  1904.  See  also  Hay's  Alternating  Currents, 
pp.  185-188. 


184      ELECTRIC  MOTORS,    THEIR   ACTION   AND   CONTROL. 


(1)  That  due  to  I1rl  which  is  in  phase  with  Ir 

(2)  That  due  to  the  leakage  reactance  of  the  primary,  i.e.,  that 
produced  by  the  winding  L1  being  in  quadrature  with  7X  and  equal 
to  o/L/j. 

(3)  That  due  to  the  mutual  flux  existing  between  primary  and 
secondary.     To  determine  the  value  of  this  we  must  consider  the 
secondary  current.     Let  its  instantaneous  value  be  i2  =  Iym  sin  tot. 
The  flux  through  the  primary  due  to  this  current  is  Mi2,  wherein  M 
is  the  coefficient  of  mutual  induction  between  primary  and  second- 
ary.    The  voltage  thereby  induced  in   the  primary,  assuming  a 
one  to  one  ratio  of  transformation,  is  —  a>MI2m  cos  cot.     To  balance 
this  the  impressed  voltage  must  have  an  opposite  component  or 

+  wMI2m  cos  a>t,  the  effective  value  of  which  is  a)MI2  in  quadrature 
with  I2.  The  total  secondary  e.m.f.  is  that  due  to  the  primary 
current,  its  value  being  ajMI^  lagging  in  quadrature  with  respect  to 
the  current  Ir  It  is  made  up  of  two  components,  one  in  phase  with 

the  secondary  current,  namely,  the 
7/2  drop,  and  one  the  leakage  reac- 
tion, (uL2I2  in  quadrature  with  72. 

These  various  voltages  are  shown 
vectorially  in  Fig.  no,  wherein  the 
primary  current  O7X  is  taken  as  the 
horizontal  axis  of  reference.  The  re- 
sistance drop  of  the  primary  is  repre- 
sented by  OA  in  phase  with  O7t;  the 
leakage  reaction  of  the  primary  is  OLV 
90°  behind  OI.  OP  is  the  induced 
e.m.f.  in  the  secondary,  due  to  the 
mutual  flux.  Its  two  components  are 
PR2  and  OR2,  corresponding  to  the 
secondary  leakage  reaction  and  resist- 
ance drop  respectively.  The  compo- 
nent of  the  primary  applied  voltage, 
FIG.  no. — VECTOR  DIAGRAM  OF  due  to  the  mutual  inductive  reaction, 

VOLTAGES    PER    PHASE    OF    »-    .       L(,    perpendicular    to    QR2.         The 
DUCTION   MOTOR. 

impressed  primary  voltage  is  then  the 

vector  resultant  of  OA,  OL^  and  L£,  or  it  is  represented  by  the 
vector  OC  at  an  angle  CO  A  or  ^  ahead  of  the  primary  current, 
the  cosine  of  which  represents  the  power  factor  of  the  motor.  The 


POLYPHASE  INDUCTION  MOTORS.  185 

angle  POR2  corresponding  to  (f>2  is  the  phase  angle  between  the 
secondary  voltage  and  current.  From  C  draw  a  line  parallel  to  OR2, 
let  this  intersect  ALl  at  N.  This  construction  gives  us  two  similar 
triangles,  namely,  OER2  and  NLfi,  wherein  Lf  and  ER2  are  pro- 
portional. 

Now  divide  each  vector  by  Ilt  thus  fixing  the  points  A,  Ll  and  P 
in  position,  because  now  the  corresponding  vectors  represent  Rit 
a)Lv  and  a>M  which  are  of  constant  value.  Hence  as  the  secondary 
current  varies,  the  angle  OR2P  being  a  right  angle,  the  point  R2 
must  describe  a  semicircle  about  OP  as  a  diameter.  The  triangle 
NL£,  however,  is  similar  to  OPR2,  so  that  any  change  in  the  latter 
must  be  accompanied  by  a  corresponding  change  in  the  former, 
or  the  point  C  must  describe  a  semicircle  on  Z^AT  as  a  diameter. 

The  point  O,  being  the  origin  of  axes,  is  fixed  in  position;  accord- 
ingly it  follows  that  OF  X  OC  is  constant  for  all  values  of  C*, 
which  may  be  expressed  as  OF  X  OC  =  K  or  OV  =  K  +  OC. 

Tf  TT 

However,  by  construction  OC  =  —  ;  consequently  O  F  =  —  /,.     The 

voltage  E  is  constant,  therefore  OF  is  directly  proportional  to  the 
primary  current,  and  we  have  the  important  fact  that  the  extremity 
of  the  vector  representing  the  primary  current  moves  along  an  arc 
of  a  circle  as  the  load  of  the  motor  changes.  With  this  rule  estab- 
lished, we  can  construct  particular  circle  diagrams  adaptable  to 
practical  use.  We  shall  employ  the  circle  diagram  proposed  by 
A.  S.  McAllister  in  the  "  Electrical  World  "  of  April  and  May,  1906.1 
For  the  construction  of  this  diagram  the  following  readings  must 
be  determined,  namely,  voltage,  current  and  watts  with  motor 
running  without  load,  and  voltage,  current  and  watts  with  its  rotor 
locked,  also  the  resistance  of  each  primary  phase  winding. 

The  equivalent  single-phase  current  is  obtainable  from  the  no  load 
ammeter  reading,  and  the  equivalent  single-phase  locked  current 
is  derived  from  the  locked  conditions.  The  equivalent  single-phase 
resistance  of  the  stator  can  be  calculated  if  resistance  per  phase 
winding  is  known.  The  reason  for  using  single-phase  equivalents 
is  that  the  circle  diagram  when  thus  constructed  gives  directly  the 
true  motor  input,  torque  and  output. 

*  The  area  of  the  rectangle  constructed  upon  any  total  secant  and  its  external 
part  is  equal  to  the  square  of  the  corresponding  tangent. 

t  Alternating  Current  Motors,  A.  S.  McAllister,  p.  109,  1909. 


186       ELECTRIC  MOTORS,    THEIR   ACTION   AND   CONTROL. 

The  equivalent  single-phase  current  in  the  case  of  two-phase 
circuits  is  the  sum  of  the  current  in  both  phases,  while  in  the  three- 
phase  system  the  equivalent  current  is  v7^  /,  where  I  is  the  average 
of  the  currents  in  each  line.  The  equivalent  single-phase  resistance 
for  any  two-phase  or  three-phase  system,  when  considering  the  like 
currents,  is  one-half  that  measured  between  phase  lines  by  direct 
currents.* 

The  watts  input  and  current  for  the  locked  condition  cannot  be 
obtained  safely  with  rated  line  voltage  because  of  the  danger  of 
damaging  the  motor  by  the  large  current  which  then  flows.  In 
practice  a  locked  saturation  curve  is  obtained  by  plotting  a  series 
of  four  or  five  readings  of  current,  power  and  torque  with  the  test 
voltage  at  rated  frequency,  and  varied  between  one-fifth  and  about 
three-fifths  of  the  operating  pressure  employed  as  abscissa.  The 
various  curves  are  then  continued  beyond  the  test  points  by  exter- 
polation.  A  rough  approximation  of  the  locked  current  and 
watts  can  be  made  by  testing  at  one-half  rated  voltage,  and  then 
multiplying  the  current  by  two  and  the  watts  by  four,  but 
possible  change  in  saturation  is  likely  to  introduce  an  error  of 
large  value,  especially  in  the  power-f actor. t  The  above  curve 
method  is  therefore  preferable,  although  it  is  evidently  open  to 
some  question.  A  series  of  locked  saturation  curves  of  a  three- 
phase  8-pole,  6o-cycle,  2i5-volt  20- h. p.  induction  motor  are  illus- 
trated in  Fig.  in. 

Construction  of  Diagram. — Let  the  vertical  line  OE,  Fig.  112, 
represent  the  line  voltage  drawn  to  scale.  Draw  at  their  proper 
phase  positions,  also  to  scale,  the  equivalent  single-phase  no  load 
and  locked  currents  (OM  and  OF),  respectively.  Through  M  draw 
a  line  OK  perpendicular  to  OE,  join  M  and  F',  draw,  also,  a  line 
perpendicular  to  the  middle  of  MF,  intersecting  OK  at  X.  With 
X  as  center  and  either  XM  or  XF  as  a  radius,  describe  the  arc 
MCF\  this  is  the  locus  of  the  primary  current.  The  distance  HG 
represents  the  added  primary  or  stator  loss  existing  with  rotor 
locked,  its  length  =  (added  primary  copper  loss  -5-  total  locked 
watts)  X  IF.  Draw  the  line  GM.  With  this  construction  com- 
pleted, the  performance  of  the  machine  may  be  determined  directly 

*  A.  S.  McAllister,  Alternating  Current  Motors,  pages  13,  14,  15, 
f  The  rotor  should  be  allowed  to  rotate  very  slowly  during  this  test,  or  the  position 
of  the  rotor  should  be  varied  and  the  results  averaged. 


POLYPHASE   INDUCTION   MOTORS. 


187 


& 

200 


150 


100 


50 


-400 


100 


200 


FIG.  III. LOCKED  SATURATION  CURVE,  2O-H.P.,  2I$-VOLT,  INDUCTION  MOTOR. 


by  inspection.     For  example,  the  factors  indicating  the  operation 
of  the  motor  with  a  current  P  are  as  follows : 

OP  to  scale  represents  the  equivalent   single-phase  primary 
current. 

Cos  POE  equals  power  factor  of  motor. 

MP  equals  secondary  current  in  primary  equivalents. 

PT  equals  primary  input  in  watts. 

TS  equals  no  load  losses  in  watts. 

RT  equals  total  primary  loss  in  watts. 

PR  equals  total  secondary  input  in  watts. 

RS  equals  the  added  primary  copper  loss. 

QR  equals  secondary  copper  loss. 

QP  equals  motor  output  in  watts. 


188       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

M 


QP  +  PT  equals  motor  efficiency. 

QR  -r-  PR  equals  the  per  cent  rotor  slip. 

QP  X  52^0        OP  X  V.CK 
Torque  =  ^  f      J  °    =  -^—       '    ° 
746  r.p.m.  r.p.m. 

OM'   =  magnetizing  current. 
M'T    =  leakage  current. 


POLYPHASE   INDUCTION  MOTORS 


189 


OM'    -T-  OP  =  per  cent  magnetizing  current. 
M'T   H-  OP  =  per  cent  leakage  current. 

Maximum  torque  is  CG',  the  point  C  being  obtained  by  drawing 
a  radius  perpendicular  to  MG. 

Maximum  output  is  BJ,  the  point  B  being  obtained  by  drawing 
a  radius  perpendicular  to  MF. 

Maximum  power  factor  exists  when  primary  current  vector  is  a 
tangent  to  the  arc,  corresponding  to  point  P  in  the  diagram. 


90      900 


150  200 

Torque  in  Ft.  Lbs. 

FIG.  113. CHARACTERISTIC  CURVES  OF  A  6O-CYCLE,  2I5-VOLT,  2O-H.P. 

INDUCTION  MOTOR. 

The  characteristic  curves  in  Fig.  113  are  those  of  a  three-phase, 
6o-cycle,  8-pole,  2i5-volt  induction  motor  of  20  horsepower  capacity, 
the  values  for  the  construction  of  these  curves  being  obtained  from 
the  circle  diagram  just  given.  The  fundamental  data  employed 
were  derived  from  test  and  are  as  follows: 


No  Load 

Values. 

Locked  Values, 
(See  Fig.  112.) 

Volts 

215 

215 

Equivalent  single-phase  amperes    

33  5 

470 

Total  watts    

930 

43  X  103 

Power  factor  

12.9% 

46.5% 

Hot  resistances  of  stator: 


Ph  (1  -  3)  =  .107  w;  Ph  (3  -  2)  =  .109  «; 
Ph(2-  1)  =  .106  w 
108 


Equivalent  single-phase  resistance  =  '—-  =  . 054  w 


190       ELECTRIC  MOTORS,    THEIR   ACTION   AND   CONTROL. 

Construction.  —  Draw  the  power  factor  circle  Fig.  112  with  radius 
of  2.5  in.  (.25  in.  =  10  per  cent  p.f.). 

Lay  off  p.f.  =  12.9  per  cent  and  draw  no  load  current  =  .335  in. 
(i  in.  =  100  amperes). 

Lay  off  p.f.  =  46.5  per  cent  and  draw  locked  current  MF  =  4.7  in. 

Join  F  and  M,  draw  MK  perpendicular  to  voltage  line  OE. 
Bisect  MF  and  erect  a  perpendicular  at  this  point.  The  inter- 
section of  this  perpendicular  with  OK  at  point  X  is  the  cen- 
ter of  current  locus.  Draw  an  arc  through  M  and  F  with  X  as 
center. 

Determination  of  motor  performance  at  load  corresponding  to 
current  OP. 

OP  =  1.33  in.  =  133  amperes;  continue  OP  until  it  intersects 
power  factor  circle,  then  project  intersection  to  power  factor  ordi- 
nate  =  2.22  in.,  or  2.22  -r-  2.5  =  88.8  per  cent. 

Line  MG  is  drawn  as  follows: 

FI    =  total  input  at  starting  motor  =  43  kw. 
HI  =  MM'  =  no  load  input  =  930  watts. 
HF  =  secondary  copper  loss  at  starting  +  increase  of  primary 
copper  loss  for  current  OF,  or  470  amperes. 

Primary  copper  loss  at  starting  with  line  voltage  =  47o2  X  '- 

2 

=  12  kw. 
No  load  primary  copper  loss  =  33-S2  X  -    --=.6  kw. 

Increase  in  primary  copper  loss  at  starting  =  n.4kw. 
The  line  FI  =  43  kw.  =  2.15  in.  or  20  kw.  =  i  in.;  thus  dis- 
tance HG  or  added  prirnary  loss  =  ^^  =  .57  in.,  which  determines 

20. o 

position  of  G,  and  from  it  MG  is  drawn. 

The  slip  at  load  corresponding  to  current  P  (i.e.,  133  amperes) 

12  3  6oO 

is  QR  -H  RP  =  -    -  =  10.9  per  cent.     Synchronous  speed  =  — 
i.io  4 

=  900  .*.  speed  =  900  (i.oo  —  .109)  =  802  r.p.m. 
Motor  input  =  TP  =  1.18  in.  or  1.18  X  20  =  33.6  kw. 
Motor  output  =  PQ  =  .98  in.  or  .98  X  20  =  19.6  kw.  =  26.3  h.p. 
Motor    efficiency  =  PQ  -J-  TR  =  19.6  -*-  23.6  =  83.1    per   cent. 


POLYPHASE   INDUCTION   MOTORS. 


191 


,,  ,  kw.  output  X  7.o<;        iQ.6  X  7.05  lu 

Motor    torque  =  *• '— ?  =   -^—   _i_?  =172    Ibs. 

r.p.m.  r.p.m. 

at  a  ft.  radius.  Since  the  torque  corresponding  to  current  OP  is 
by  calculation  172  ft.  Ibs.,  and  the  vector  RP  corresponding  thereto  is 
1. 12  in.  long,  we  can  state  that  i  in.  on  the  torque  lines  of  Fig.  113 
is  equivalent  to  an  effort  of  153  Ibs.  at  a  foot  radius. 

Per  cent  magnetization  current  =  OM  +  OP  =  -&-  =  24.8  per 
cent. 

Per  cent  leakage  current  =  M'T  -*-  OP  =  — -  =  20.3  per  cent. 

oo 

DATA  FOR    CHARACTERISTIC    CURVES  OF  A    20-H.P.   INDUCTION 
MOTOR  DERIVED  FROM  CIRCLE  DIAGRAM,  FIG.  112. 


Point. 

Equi. 
Pri- 
mary 
Amps. 

Slip 
OR/PR  =  S  % 

R.P.M. 

Alts.  (1-s) 

Effic. 
PQ/PT  =  % 

Output. 
I"  =  26.8 
H.P. 
PQ" 
H.P. 

Torque. 
I"  =  153 
PR 
Ft.  Lbs. 

P.F. 

poles 

005 

1" 

Af\ 

•  WWW   _ 

070 

7fi    ^ 

IftK  A     A 

IftC  OC     0 

RO     f\ 

t\J 

.165  "" 

o  /  O 

.220     76'8 

.  1  UO  —     TI  .  rr 

.  1  OO  —     ^JO  .  o 

OO  .  U 

CO 

O     £ 

8fi7 

.32 

OO               0     ft 

OC                   Cfi     A 

70    n 

Do 

.36    '       3'6 

oO  i 

.38 

.  o^     —     O  .  0 

.  OO    —     OO  .  U 

/  O  .  U 

3 

77 

Ts  =    5<4 

851 

.52 
760 

.52  =14.0 

.55  =  86.0 

83.0 

4 

97 

^Ts  =    7'7 

830 

ti  =86-5 

.73  =19.5 

.78  =122.0 

87.0 

P 

133 

FI5  =  10-9 

802 

ril  =83-1 

.98  =26.3 

1.10  =172.0 

89.0 

6 

174 

.19 

778 

1:20  -79  0 

1.2     =32.2 

1.40  =218.0 

88.0 

1.140 

1.52       79'° 

7 

228 

•34    =   19  3 

725 

|42  =73.0 

1.42  =38.0 

1.76  =275.0 

85.0 

1  .  76 

1.95 

B 

277 

•52    -   26  0 

666 

^°    ««   n 

1.48  =39.0 

2.00  =312.0 

81.0 

2.00    ™   26'° 

2.25 

q 

Q9.n 

.75 

coc 

1>38  -56  5 

i    qc    0,7   f» 

21  q          QQ9    ft 

7K    A 

*7 

oou 

2.13 

OuO 

2.46 

1  .  oo    —  o  i  .  U 

.  lo  .  —  oo*  .  U 

1  O  .  U 

10 

349 

215    =   41<0 

530 

1-26  =50.0 

1.26  =33.8 

2.20  =340.0 

72.0 

11 

376 

2~rf  =  44-3 

500 

r^  =45-5 

1.16  =31.0 

2.15  =336.0 

68.0 

12 

425 

1.25      _   64 

324 

o-4-n  =29.0 

.72  =19.3 

1.96  =306.0 

58.0 

1.96 

2.50 

13 

445 

1-38    =76.5 

212 

•42  -17  6 

.42  =11.3 

1.80  =281.0 

54.0 

n     net            If  «  V 

.80 

2.38 

14 

457 

^75  =  86<0 

126 

2T5  =1°'7 

.24  =   6.4 

1.70  =265.0 

51.0 

15 

470 

-fo  -100'0 

0 

0 

0        0 

1.50  =234.0 

46.5 

192       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

Maximum  motor  torque  CGf   =  2.16  X  176  =  340  ft.  Ibs. 
Maximum  motor  output  =  1000  (BJ  X  20)  -r-  746  =  30.0  -*-  .746 
=  40.2  horsepower. 

The  performances  for  different  current  values  corresponding 
to  a  series  of  points  indicated  on  the  circle  diagram  were  simi- 
larly obtained,  and  for  convenience  of  reference  are  arranged 
in  the  preceding  table,  page  191,  to  which  the  curves  in  Fig.  113 
correspond. 

The  speed  regulation  of  this  particular  motor  is  fairly  good  up 
to  150  per  cent  of  rated  torque,  beyond  which  limit  the  drop  in  speed 
becomes  pronounced,  and  at  a  torque  of  340  ft.  Ibs.  (2.7  times  rated 
value)  the  motor  reaches  its  "pull  out  torque."  The  "pull  out" 
limit  (i.e.,  maximum  torque  developed)  of  an  induction  motor  is 
that  point  upon  its  speed-torque  curve  at  which  any  attempt  to 
further  increase  the  torque  causes  the  motor  to  drop  rapidly  in 
speed  and  stop.  This  characteristic  is  very  pronounced  in  induction 
motors,  the  exact  location  of  this  point  depending  largely  upon  the 
flux  leakage  occurring.  It  usually  varies  between  two  and  three 
times  the  rated  torque,  depending  upon  the  size  of  the  motor,  and 
may  be  obtained  at  starting  if  the  rotor  resistance  and  "standstill" 
reactance  are  made  equal  (p.  181).  The  maximum  horsepower 
output  of  this  induction  motor  is  obtainable  at  a  speed  greater  than 
that  existing  at  its  maximum  torque,  and  this  is  usually  the  case  with 
electric  motors.  The  power  factor  curve  indicates  one  of  the  diffi- 
culties caused  by  induction  motor  loads,  namely,  the  production  of 
a  wattless  current,  which  is  particularly  pronounced  at  light  loads. 
The  power  factor  of  an  induction  motor  increases  with  the  load  to 
nearly  the  "pulling  out"  point,  after  which  it  decreases,  and  unless 
a  special  method  be  employed  to  secure  maximum  torque  at  starting, 
the  power  factor  at  standstill  is  usually  much  lower  than  when  run- 
ning at  or  near  rated  load. 

The  following  table  gives  the  characteristics  of  operation  attained 
by  standard  machines,  and  it  should  be  noted  that  the  power  factor 
increases  somewhat  with  the  size  of  the  motor. 

Starting  torque  and  pull  out  torque  are  in  terms  of  rated  load 
torque. 

Starting  amperes  equal  amperes  to  start  with  rated  load  torque 
at  line  voltage,  in  terms  of  rated  load  current. 


POLYPHASE   INDUCTION   MOTORS. 


193 


DATA  OF  STANDARD   INDUCTION  MOTORS. 
Machines  Two-  and  Three-phase  for  100  to  550  volts. 


H.P. 

Poles. 

Slip 
Per 
Cent. 

Amps. 
Start. 

Start 
Torque 

Pull 
Out 

Torque 

Per  cent  Power  Factor. 

Per  cent  Efficiency. 

i 

Load. 

Load. 

Rated 
Load. 

H 
Load. 

* 
Load. 

I 
Load. 

Rated 
Load. 

1 
Load. 

| 

4 

6.7 

1.5 

1.7 

2.0 

57 

66 

72 

72 

72 

74 

74 

72 

1 

4 

7.0 

2.1 

2.2 

2.4 

68 

73 

78 

79 

75 

78 

79 

77 

2 

4 

6.0 

2.3 

.9 

2.6 

69 

78 

83 

83 

80 

82 

82 

80 

3 

4 

7.4 

2.5 

.5 

2.2 

75 

83 

86 

87 

82 

84 

83 

82 

5 

4 

8.5 

2.3 

.7 

2.2 

76 

84 

88 

88 

83 

84 

85 

83 

7.5 

6 

7.4 

2.3 

.9 

2.5 

75 

83 

87 

87 

83 

85 

85 

84 

10 

6 

6.8 

2.5 

.8 

2.3 

78 

86 

88 

88 

84 

85 

85 

83 

15 

6 

7.3 

2.2 

.9 

2.5 

80 

86 

89 

89 

85 

86 

86 

84 

20 

6 

7.2 

2.3 

.8 

2.3 

82 

87 

90 

90 

85 

86 

86 

83 

20 

8 

8.0 

2.7 

.0 

2.7 

78 

85 

88 

89 

86 

87 

86 

84 

25 

6 

7.0 

2.2 

.8 

2.3 

80 

87 

88 

89 

85 

87 

87 

84 

30 

8 

6.7 

2.7 

.8 

2.4 

83 

88 

90 

90 

87 

87 

86 

85 

40 

8 

6.5 

2.6 

2.8 

2.8 

80 

86 

89 

90 

87 

87 

87 

85 

50 

8 

6.2 

2.5 

2.3 

3.0 

82 

88 

91 

91 

87 

88 

88 

86 

75 

10 

5.6 

2.6 

2.3 

3.1 

78 

84 

88 

89 

87 

88 

88 

87 

For  further  information  upon  theory  and  construction  of  induction  motors,  the 
reader  is  referred  to  the  following  standard  works: 
ALTERNATING  CURRENT  MOTORS.    A.  S.  McAllister.    New  York,  1909. 
ALTERNATING  CURRENT  PHENOMENA.    C.  P.  Steinmetz.    New  York,  1908. 
COURANTS  ALTERNATES,  Vol.  II.     G.  Sartori.     Paris,  1905. 
DYNAMO-ELECTRIC  MACHINERY,  Vol.  II.    S.  P.  Thompson.     London,  1905. 
ELECTRIC  MOTORS.     H.  M.  Hobart.     London,  1904. 
ELECTRIC  TRANSMISSION  OF  ENERGY.     Gisbert  Kapp. 
THE  INDUCTION  MOTOR.    Behrend.    New  York,  1903. 
THE  INDUCTION  MOTOR.     De  la  Tour-Mailioux.     New  York,  1904. 
WECHSELSTROM-TECHNIK,  Vol.  V.     E.  Arnold.     Berlin,  1909. 


CHAPTER  XV. 

STARTING  OF  INDUCTION  MOTORS. 

THE  fact  that  an  induction  motor  is  substantially  a  transformer 
with  a  short-circuited  secondary  causes  difficulty  in  starting, 
especially  when  its  terminals  are  directly  connected  to  full  line 
pressure.  For  example:  The  locked  saturation  curves  of  an 
induction  motor,  as  shown  in  Fig.  in  (p.  187),  indicate  that  direct 
application  of  the  full  line  pressure  to  the  stator  terminals,  with  the 
rotor  short-circuited  and  standing  still,  produces  an  inrush  primary 
current  which  is  nearly  five  times  rated  value.  Such  excessive  cur- 
rent is  likely  to  injure  the  insulation  of  the  windings  and  should  be 
avoided.  In  addition  to  this,  the  power  factor  of  this  current  is  very 
low,  being  about  thirty  to  forty  per  cent.  It  also  affects  the  line 
regulation,  causing  voltage  fluctuation.  Consequently,  when  the 
motor  to  be  started  is  of  even  moderate  size  (over  3  h.p.)  some  means 
should  be  employed  to  limit  the  line  current  at  starting  to  reasonable 
values. 

Two  general  forms  of  rotor  windings  are  employed  in  practice, 
and  as  a  result  two  methods  of  starting  have  been  developed  which 
depend  respectively  upon: 

(a)  Reduction  of  Line  Voltage. 

(b)  Resistance  Control. 

Starting  by  means  of  reduced  line  voltage  is  necessary  when 
squirrel-cage  rotors  are  employed,  and  it  is  generally  accomplished 
through  the  introduction  of  an  auto -transformer  or  compensator 
into  the  primary  circuit.  The  underlying  principle  of  this  type  of 
starter  will  be  understood  by  referring  to  Fig.  114.  The  device  is 
equivalent  to  a  single-coil  step-down  transformer,  the  ratio  of  trans- 
formation being  that  existing  between  the  total  number  of  turns 
across  which  the  primary  terminals  are  connected  and  those  between 
which  the  load  is  placed.  In  the  specific  instance  illustrated  in 
Fig.  114,  the  primary  potential  is  440  volts,  the  secondary  voltage 
is  176,  secondary  current  200  amperes,  and  primary  current  80 
amperes.  The  voltage  across  the  stator  terminals  is  only  a  frac- 

194 


STARTING   OF  INDUCTION  MOTORS. 


195 


tion  of  the  line  potential,  when  the  switch  is  placed  in  the  starting 
position,  but  after  the  motor  has  approximately  reached  its  rated 
speed,  the  switch  is  thrown  over  rapidly  into  the  running  position, 
the  stator  being  then  directly  connected  to  the  supply  voltage. 

200  Amps. 

80  Amps. 


C^ 

1 1 


FIG.  114. SIMPLE  AUTO-TRANSFORMER  CONNECTIONS. 

The  compensator  windings  for  a  three-phase  motor  consist  of  three 
coils,  one  for  each  phase,  each  coil  being  placed  upon  a  separate  leg 
of  a  laminated  iron  core.  Each  coil  is  provided  with  three  or  more 
taps,  so  that  a  number  of  sub-voltages  may  be  obtained,  any  one  of 
which  may  be  selected  for  permanent  connection  to  the  throw-over 


Running 


Starting 


FIG.  115. CONNECTIONS  OF  STARTING  COMPENSATOR  FOR  THREE-PHASE 

INDUCTION  MOTOR. 

switch,  according  to  service  conditions.  The  three  coils  of  the  com- 
pensator are  Y-connected,  the  supply  line  to  the  three  free  ends  and 
the  starting  connections  of  the  motor  to  the  taps  being  as  shown  in 
Fig.  115.  To  meet  various  requirements,  compensators  are  gen- 
erally provided  with  taps  giving  potentials  approximately  equal  to 


196       ELECTRIC  MOTORS,    THEIR   ACTION   AND   CONTROL. 


40,  58,  70  and  80  per  cent  of  the  line  voltage,  though  the  70  per. cent 
value  meets  most  of  the  commercial  requirements,  as  it  gives  prac- 
tically full  load  torque  for  starting.  The  line  currents  with  the 
above  taps  are  respectively  16,  34,  50  and  64  per  cent  of  that  which 
would  be  drawn  by  the  motor  if  no  compensator  were  employed. 
The  chief  objection  to  the  compensator  is  its  costj  being  about  25 
per  cent  of  that  of  the  motor.  It  has  been  suggested  that  this 
expense  could  be  reduced  by  using  one  compensator  for  starting  a 
number  of  motors,  the  method  recommended  being  as  follows :  * 
A  throw-over  switch  is  provided  for  each  motor  to  be  started,  and  a 
three-pole  compensator  supply  switch.  Only  one  motor  can  be 
started  at  a  time,  thus  avoiding  the  line  disturbance  caused  by 
simultaneous  starting  of  two  or  more  motors,  each  motor  switch 
being  thrown  into  the  running  position  as  soon  as  the  machine  has 


Supply  Bus  Bars 


r  \     \  . 

\ 


C.B. 

Funning  Side 

To  Motor  No.  3 

Starting  Side 


Compensator 
Coils 


FIG.  Il6. CONNECTIONS  FOR  STARTING  SEVERAL  MOTORS  BY  MEANS  OF 

ONE  COMPENSATOR. 

approximately  reached  its  normal  speed.  When  all  motors  have 
been  started,  the  compensator  supply  switch  should  be  opened. 
The  diagram  (Fig.  116)  shows  the  method  of  connecting  three  motors 
to  one  compensator. 

*  G.   Stevenson,    Journal   Institution  of  Electrical  Engineers,  Vol.   XLI,  1908, 
p.  685. 


STARTING  OF  INDUCTION  MOTORS. 


197 


Three-phase  motors  may  be  started  without  a  compensator,  by 
Y-connecting  the  stator  windings  at  starting,  and  employing  delta 
connections  for  running,  the  change  being  rapidly  made  by  means 
of  a  special  throw-over  or  double-throw  four  point  switch.  The 
connections  for  such  a  starting  scheme  are  as  illustrated  in  Fig.  117. 

Motor  Windings 


1 

\    1 

rrr 

—  ^ 

Starting     C  Q 

IT-  L^r 

Q~     jfl3 

D-J     a 

i 

rr 

IT 
n 

!       Q 

LJ 

i 

Supply  Lines      L 

[ 

Running     BJ 

r 

]  [ 

rj  [ 

]  A[ 

]       [ 
rj     t 

i                        n 
Jj              ,-1 

r 

h 

LL 

—  ^-H  

3 

FIG.  117. CONNECTIONS  FOR  STARTING  THREE-PHASE  INDUCTION  MOTOR. 

STATOR  Y-CONNECTED  FOR  STARTING. 

By  this  method  the  voltage  per  phase  at  starting  is  only  i  -J- 
or  58  per  cent  of  the  line  voltage.  It  follows,  then,  that  the  starting 
current  and  torque  are  also  reduced.  For  example,  consider  the 
2o-h.p.  motor  already  referred  to;  the  starting  current  employ- 
ing Y-connection  on  starting  would  be  only  one-third  of  that 
taken  if  the  motor  were  thrown  directly  on  the  line  with  delta- 
connected  stator,  or  it  would  be  (470  -^  97)  •*-  3  =  1.62  times  full 
load  current.  The  starting  torque  being  proportional  to  the  square 
of  the  potential  difference  employed,  would  give  a  value  of  torque 
equal  to  one-third  of  the  value  obtained  with  full  line  voltage. 

Boucherot  Method.  —  An  excellent  method  for  starting  induction 
motors  provided  with  squirrel-cage  rotors  is  that  devised  by  M.  P. 
Boucherot.*  The  general  scheme  is  to  employ  the  ordinary  form 
of  stator  as  the  primary,  and  to  provide  a  rotor  with  several  squirrel- 
cage  windings  of  graded  resistance  and  reactance  varying  from 
high  resistance  with  low  inductance  to  low  resistance  with  high 

*  Bulletin  de  la  Societe  International  des  ^lectriciens,  February,  1898,  and  Electric 
Motors,  H.  M.  Hobart,  pp.  266-270,  London,  1904. 


198       ELECTRIC  MOTORS,    THEIR   ACTION  AND   CONTROL. 

inductance.  The  high  resistance  circuits  are  the  seats  of  large 
induced  currents  at  starting,  while  those  of  high  inductance  have  only 
small  currents,  because  at  standstill  their  reactance  is  high.  The 
starting  is  due  to  the  high  resistance  windings.  As  the  rotor  speeds 
up  from  standstill,  the  frequency  of  the  secondary  e.m.f.  decreases; 
consequently  the  reactance  of  the  windings  diminishes,  and  all 
circuits  carry  current,  that  of  the  highly  inductive  circuits  becoming 
relatively  larger,  because  their  resistance  is  extremely  low.  Thus  by 
this  method  the  advantages  of  a  high  resistance  rotor  for  starting 
are  secured,  while  the  poor  speed  regulation  and  low  efficiency  of 
such  a  winding  under  varying  load  are  avoided  by  the  fact  that  the 
low  resistance  (high  reactance)  windings  are  the  working  ones. 

A  double  squirrel-cage  winding  is  usually  found  to  be  sufficient 
to  meet  practical  requirements,  Fig.  118  showing  a  rotor  punching 


12  13 

FIG.  1 1 8. ROTOR  LAMINATIONS  OF  A  BOUCHEROT  MOTOR. 


of  such  a  motor.  The  radial  openings  joining  the  upper  and  lower 
slots  are  designed  to  prevent  the  occurrence  of  excessive  magnetic 
leakage  with  respect  to  the  inner  winding.  Copper  bars  are  placed 
in  the  outer  series  of  holes,  and  these  are  connected  by  means  of 
high  resistance  end  rings  formed  of  German  silver  or  other  resistance 
alloy.  Copper  bars  of  larger  cross  section  than  those  of  the  outer 


STARTING   OF  INDUCTION  MOTORS. 


199 


group  are  placed  in  the  inner  series  of  slots,  and  these  are  secured 
to  low  resistance  end  rings. 

The  speed-torque  curves  of  such  a  motor  are  illustrated  in  Fig. 
119;*  of  these,  curve  A  represents  the  action  when  the  motor  is  op- 


60         80        100       120       140       160       180       200 
Percent  Rated  Torque 


0         20         40 
FIG.  IIQ.  -  SPEED-TORQUE  CURVES  OF  A  BOUCHEROT  INDUCTION  MOTOR. 


crated  with  only  the  outer  or  high  resistance  winding  active.  In  this 
case  the  starting  torque  available  is  nearly  twice  that  at  rated  load, 
and  the  slip  at  rated  load  is  about  25  per  cent.  Curve  B  indicates 
the  speed-torque  relations  when  the  inner  or  highly  reactive  winding 
only  is  used.  Under  this  condition  the  motor  has  practically  no 
starting  torque,  while  the  maximum  available  torque  when  running 
is  only  60  per  cent  of  the  rated  value,  and  the  corresponding  slip  is 
6  per  cent.  The  speed  torque  characteristic  of  the  motor  with 
both  windings  active  is  shown  in  curve  C.  The  starting  torque 
then  obtained  is  substantially  twice  that  existing  at  rated  load. 
The  speed  regulation  is  excellent,  a  slip  of  but  6  per  cent  occurring 
at  rated  load. 

It  is  surprising  that  this  method  of  control  is  not  more  widely 
employed,  since  the  efficiency  of  the  motor  thus  designed  is  high, 
the  starting  torque  good,  and  the  control  extremely  simple,  all  that 
is  necessary  to  start  the  motor  being  a  simple  closing  of  an  ordinary 
supply  switch. 

Resistance  Control.  — It  was  proven  in  the  discussion  of  the 
torque  equation  of  the  induction  motor  (p.  181)  that  the  starting 
torque  of  this  type  of  machine  may  be  varied  by  changing  the  resist- 
ance of  its  secondary  winding.  With  this  method  of  control  the 
starting  torque  can  be  made  anything  up  to  the  maximum  value, 

*  Electric  Motors,  H.  M.  Hobart,  p.  269,  London,  1904. 


200       ELECTRIC  MOTORS,    THEIR   ACTION  AND   CONTROL. 

that  is,  two  or  three  times  the  rated  load  torque.  In  the  case  of 
small  machines  (3  to  5  horsepower),  in  which  no  speed  regulation 
is  required,  provision  may  be  made  to  locate  the  special  resistance 
grids  in  the  annular  space  between  rotor  core  and  shaft,  employ- 
ing for  this  purpose  an  overhung  core.  For  example,  the  three  free 
ends  of  the  rotor  winding  are  connected  to  three  resistance  grids 
placed  within  the  rotor  spider.  This  resistance  is  subsequently  cut 
out,  by  operating  a  lever  which  engages  a  collar  free  to  slip  longitudi- 
nally upon  the  shaft.  This  collar  moves  over  the  resistance  grids, 
gradually  reducing  their  value,  until  they  are  completely  short-cir- 
cuited. This  method,  while  applicable  to  small  machines,  is  not 
advisable  for  large  ones  on  account  of  excessive  PR  loss  in  the  resist- 
ances, which  if  confined  within  the  rotor  would  produce  extreme 
heating  and  perhaps  ultimately  injure  the  motor.  Consequently, 
in  large  machines,  or  in  the  case  of  those  whose  speed  is  to  be  ad- 
justed, the  regulating  resistances  are  placed  external  to  the  motor, 
connections  being  made  to  the  free  ends  of  the  Y-rotor  winding  by 
means  of  three  slip-rings  and  brushes,  Fig.  120.  This  type  of 
resistance  control,  owing  to  the  presence  of  the  slip-rings,  is  commer- 
cially known  as  the  slip-ring  method. 


b  c 


FIG.  120. CONNECTIONS  OF  SLIP-RING  STARTING  DEVICE. 

The  slip  of  an  induction  motor  at  a  given  torque  varies  directly 
as  the  secondary  copper  losses  (p.  182);  hence  if  the  rotor  resist- 
ance per  phase  winding  be  doubled,  the  slip  for  any  given  torque 
will  be  increased  100  per  cent;  if  the  resistance  be  increased  to  three 
times  its  initial  value,  the  slip  will  be  thrice  its  former  amount,  etc. 
The  curves  shown  in  Fig.  121  are  obtained  from  the  speed-torque 
curve  of  Fig.  113,  and  they  correspond  to  secondary  rotor  resistances 
of  one,  one  and  one-half,  two,  four,  five  and  eight  times  that  existing 
with  the  rotor  short-circuited.  These  externally  added  resistances 


STARTING   OF  INDUCTION  MOTORS. 


201 


are  Y-connected  and  the  movable  contact  arms  cut  out  resistance 
equally  in  each  of  the  branches,  as  shown  in  Fig.  120. 

The  amount  of  external  resistance  needed  to  obtain  any  given 
starting  torque  within  the  range  of  the  motor's  capabilities  can  be 
readily  determined  from  the  speed- torque  curve  obtained  when  the 
rotor  is  operated  with  its  windings  short-circuited.  For  example, 
it  is  desired  to  have  the  typical  motor  operate  so  that  it  will  give,  as 
a  maximum,  approximately  rated  torque  when  starting;  and  Fig.  113 
shows  that  rated  torque  exists  when  the  slip  is  eight  per  cent. 


50 


100 


150  200 

Ft.  Lbs.  Torque 


250 


300 


FIG.  121. SPEED-TORQUE  CURVES  OF  A  2O-H.P.  INDUCTION  MOTOR, 

WITH  VARIOUS  VALUES  OF  ROTOR  RESISTANCE. 


Hence  to  have  this  torque  developed  at  standstill,  the  desired 
resistance  of  the  rotor  circuit  must  be  such  as  to  increase  the  slip 
about  twelvefold.  However,  since  the  resistance  per  phase  winding 
of  the  rotor  is  .044  ohm,  approximately  .5  ohm  additional  must  be 
placed  in  each  branch.  Similarly,  if  it  be  desired  that  the  motor 
exert  the  maximum  torque  available  at  starting,  the  necessary  ex- 
ternal resistance  can  be  also  determined  directly  from  the  speed- 
torque  curve  of  Fig.  113.  The  slip  at  maximum  torque  is  40  per 
cent,  therefore  to  have  100  per  cent  slip  and  same  torque,  the  rotor 
resistance  must  be  increased  to  about  2.5  times  its  initial  value, 
that  is,  a  total  of  .044  X  2.5  =  .no  ohm  must  be  placed  in  each 
phase  circuit  of  the  rotor. 

The  advantage  of  employing  an  adjustable  resistance  in  the  rotor 


202       ELECTRIC  MOTORS,    THEIR   ACTION  AND    CONTROL. 


circuit  for  starting  a  motor  is  clearly  indicated  by  the  curves  in 
Fig.  122.  Of  these,  curve  A  shows  the  starting  current  drawn 
by  the  typical  induction  motor,  when  connected  directly  to  the  line 
without  starting  resistance  in  the  rotor  circuit.  Curve  B  shows  the 


500 
400 


ft 
4200 


100 
0 


R.P.M 


at 


b   680 


d   530 


f   750 
S 


4  6 


8        10        12        14        16        18         20        22 
TSlme  in  Seconds 


FIG.  122. STARTING  OF  2O-H.P.  INDUCTION  MOTOR,  WITH  VALUES  OF 

ROTOR  RESISTANCE. 


To  2  To         3  To  4  To  5  To          6  To          7  To          8  To 

Rotor  Resistance  in  Terms  of-the  Short  Circuit  Value 

FIG.  123. EFFECT  OF  ROTOR  RESISTANCE  UPON  THE  ACTION  OF  A 

20-H.P.  INDUCTION  MOTOR 

starting  current  existing  when  three  times  the  initial  rotor  resist- 
ance (.132  ohm  per  circuit)  is  employed  and  the  increase  of  current 
occurring  when  this  resistance  is  short-circuited  on  the  second 
step  after  twelve  seconds  acceleration.  Curve  C  shows  what 
results  when  the  added  external  resistance  is  four  times  the  rotor 


STARTING   OF   INDUCTION  MOTORS.  203 

resistance  (total  per  phase  .22  ohm),  and  is  gradually  reduced  in 
five  steps  to  its  short-circuit  value.  This  method  causes  a  very 
marked  reduction  in  the  average  starting  current,  and  is  used  where 
the  supply  circuit  must  not  be  disturbed  by  voltage  fluctuations. 

The  effects  of  adjustable  resistance  in  the  rotor  circuit  upon  power 
factor,  torque  and  primary  current  at  starting,  as  well  as  upon  the 
speeds  attained  at  rated  torque,  are  indicated  in  the  curves  of  Fig. 
123,  which  refer  to  the  2o-h.p.  motor  previously  considered.  These 
curves  show  that  addition  to  rotor  resistance  at  starting  improves 
the  power  factor,  reduces  the  starting  current,  while  it  also  in- 
creases the  starting  torque  until  the  rotor  resistance  equals  rotor 
reactance,  beyond  which  the  torque  falls  off. 

Excellent  discussions  concerning  the  various  methods  employed  for  starting  poly- 
phase induction  motors  are  given  in  the  following: 

ALTERNATING  CURRENTS.    A.  Hay.     London,  1906. 

ELECTRIC  MOTORS.     H.  M.  Hobart.     London,  1904. 

ELECTRIC  MOTORS.     N.  G.  Meade.     1908. 

HANDBUCH  DER  ELEKTROTECHNIK,  Vol.  IX.    Leipzig,  1901. 

POLYPHASE  MOTOR.     B.  G.  Lamme.     Electric  Journal,  Vol.  I,  1904. 

THE  INDUCTION  MOTOR,  CHOICE  OF  TYPE.  G.  Stevenson.  Journal  Inst.  E.  £., 
Vol.  XLI,  1908. 

WECHSELSTROMTECHNIK.     E.  Arnold.     1909. 


CHAPTER   XVI. 

SPEED  CONTROL  OF  POLYPHASE  INDUCTION  MOTORS. 

THE  induction  motor,  as  already  shown,  is  substantially  a  constant 
speed  machine.  Its  change  in  speed  beween  rated  load  and  no 
load  is  from  4  to  8  per  cent,  depending  upon  the  capacity  of  the 
machine,  the  larger  sizes  usually  having  the  better  speed  regulation. 
However,  for  many  practical  applications,  such  as  hoisting,  machine 
tool  and  traction  work,  it  becomes  advisable  to  have  a  motor,  the 
speed  of  which  may  be  adjusted  over  wide  limits.  It  is  the  object 
of  this  chapter  to  examine  the  various  methods  by  means  of  which 
the  adjustability  of  the  speed  of  an  induction  motor  can  be  secured, 
and  these  are: 

Variation  of  the  frequency  of  the  supply  voltage. 
Variation  of  the  number  of  motor  poles. 
Variation  of  the  rotor  resistance. 
Cascade  or  concatenated  connection. 
Variation  of  applied  potential. 

Frequently  combinations  of  these  methods  are  employed  in  order 
to  obtain  wider  speed  ranges,  better  regulation  or  more  gradual 
steps  of  adjustment  than  would  be  economically  possible  with  any 
single  control. 

Variation  of  Supply  Frequency.  —  The  speed  in  r.p.m.  of  the 
rotary  field  of  an  induction  motor  is,  as  already  shown,  equal  to 
60  frequency  +  pairs  of  poles;  hence  any  change  in  the  periodicity  of 
the  applied  voltage  would  be  reproduced  in  exact  proportion  in  the 
speed  of  the  rotary  field.  Thus  any  method  which  offers  a  wide 
range  of  frequency  is  theoretically  the  ideal  system  of  speed  con- 
trol; unfortunately,  however,  the  obtaining  of  such  a  source  of 
power  supply  is  not  commercially  feasible  at  present.  In  the  case 
where  but  a  single  motor  is  operated,  the  generator  speed  could  be 
altered,  and  thus  the  frequency  of  the  current.  The  voltage  should 
be  varied  as  the  frequency  when  this  method  of  control  is  em- 

204 


SPEED  CONTROL  OF  POLYPHASE  INDUCTION  MOTORS.      205 

ployed,  otherwise  the  no  load  current  would  either  be  excessive 
or  too  small,  according  as  the  frequency  is  low  or  high,  and  thus 
the  power-factor  of  the  machine  considerably  changed. 

The  induction  motor,  when  operated  in  the  manner  above  indi- 
cated, is  substantially  a  constant  torque  machine,  in  the  sense 
employed  in  this  book  (p.  47).  The  speed-torque,  current-torque, 
and  power  factor-torque  curves  of  a  2o-h.p.  motor,  when  oper- 
ated with  currents  having  frequencies  of  20,  40  and  60  cycles, 
are  respectively  as  shown  in  Fig.  124.  The  supply  voltage  is 


90 


100  150 

Ft.  Lbs.  Torque 

FIG.  124. CHARACTERISTIC  CURVES  OF  AN  INDUCTION  MOTOR  WITH 

FREQUENCY  CONTROL. 

altered  with  the  frequency;  that  is,  pressures  of  72.5,  143  and  215 
volts  are  employed.  This  group  of  curves  shows  that  the  current 
and  power  factor  for  any  given  torque  equal  to,  or  less  than,  the 
rated  value  are  practically  constant,  and  independent  of  the  fre- 
quency employed.  The  speeds  attained  at  a  given  torque  vary  in 
a  greater  ratio  than  the  frequency,  being  836  r.p.m.  at  rated  torque 
and  60  cycles,  530  r.p.m.  at  40  cycles;  and  230  r.p.m.  at  20  cycles, 
practically  a  4  to  i  range.  This  difference  is  due  to  the  departure 
of  the  machine  from  ideal  conditions,  since  resistance  and  leakage 


206       ELECTRIC  MOTORS,    THEIR   ACTION    AND   CONTROL. 

are  present.     The  regulation  becomes  poorer  as  the  periodicity  is 
lowered. 

Speed  Control  by  Changing  Number  of  Poles.  —  The  synchronous 
speed  in  r.p.m.  of  an  induction  motor  being  given  by  the  expression 
60  X  /  -j-  #  (p.  173),  it  is  evident  that  the  speed  varies  inversely 
as  the  number  of  poles.  Thus  a  motor  wound  for  six  pairs  of 
poles  and  operating  normally  at  600  r.p.m.  will  rotate  at  a  speed  of 
1200  r.p.m.  if  its  stator  winding  be  rearranged  so  as  to  have  3  pairs 
of  poles.  The  simplest  method  of  applying  this  control  is  to  em- 
ploy a  stator  having  two  or  'more  separate  windings,  corresponding 
to  different  numbers  of  poles.  One  winding  may  be  used,  it  being 
arranged  for  different  speeds  by  means  of  a  commutator  switch, 
which  alters  the  grouping  of  the  coils  and  thus  the  number  of  poles. 
A  rotor  of  the  squirrel-cage  type  is  the  only  practical  one,  because 
this  being  short-circuited  upon  itself  is  adapted  to  any  number  of 
poles.  A  grouped  or  polar  rotor  winding  requires  a  rearrangement 
of  its  coils  in  the  same  order  as  those  of  the  stator,  though  two  or 
more  independent  rotor  windings  could  be  used. 

The  connections  of  a  multi-speed  motor  of  this  type  are  relatively 
simple,  especially  if  only  a  two  to  one  speed  step  is  required  and  the 
rotor  is  of  the  cage  type.  In  such  instance,  only  six  leads  are 
brought  out  from  the  machine  if  it  is  for  three-phase  circuits,  and 
eight  when  for  two-phase  lines.  However,  should  a  polar  rotor 
winding  be  used  (to  allow  for  slip-ring  control),  twelve  leads  must 
be  brought  out  from  the  machine  if  it  is  for  three-phase  connection, 
six  of  these  terminals  being  for  the  stator  winding  and  the  remainder 
for  the  rotor.  Similarly  if  a  three  to  one  speed  adjustment  (in  three 
steps)  were  wanted,  three-pole  groupings  would  be  required,  and 
eighteen  leads  would  be  brought  out  from  the  motor,  nine  for  the 
stator  and  rotor  respectively.  Consequently  this  method  of  con- 
trol is  objectionable  as  regards  complication  of  connections  when 
more  than  a  two  to  one  speed  is  desired,  especially  in  the  case  of 
machines  having  wound  rotors.  A  further  criticism  is  that  the 
speed  changes  can  be  made  only  by  opening  and  closing  the  con- 
nec+i.ons  to  the  supply  lines,  which  as  already  shown  (p.  194)  is 
very  likely  to  cause  wide  variations  in  the  primary  current  and  fluc- 
tuations in  the  line  voltage. 

The  power  factor  of  this  type  of  multi-speed  machine  is  not  greatly 
affected  by  change  in  the  number  of  poles,  though  it  is  somewhat 


SPEED  CONTROL  OF  POLYPHASE  INDUCTION  MOTORS.      207 

higher  with  the  smaller  number.  The  efficiency  and  speed  regu- 
lation are  better  with  the  greater  number  of  poles.  This  method  for 
adjusting  the  speed  of  an  induction  motor,  when  a  two  to  one  range 
is  desired,  is  undoubtedly  the  most  satisfactory  as  regards  efficiency 
and  excellence  of  speed  regulation.  High  first  cost  and  the  need 
of  disconnecting  it  from  the  line  to  change  the  speed  are  the  ob- 
jections which  operate  against  its  general  adoption. 

Variation  of  Resistance  of  Rotor  Winding.  —  The  third  method 
of  adjusting  the  speed  of  an  induction  motor  is  by  varying  the  resist- 
ance of  the  rotor  winding.  This  arrangement  has  already  been 
considered  under  the  heading  of  slip-ring  control,  and  curves  show- 
ing the  effect  of  resistance  in  the  rotor  are  given  on  pp.  201,  202. 
It  does  not  give  a  constant  or  even  approximately  constant  speed 
over  the  torque  range.  The  speed  changes  occurring  upon  vari- 
ation of  torque  are  very  marked,  and  depend  upon  the  value  of 
the  external  resistance  employed,  the  character  being  as  shown  in 
the  curves  of  Fig.  121.  The  speed  regulation  is  comparable  to  that 
of  a  d.  c.  shunt  motor  having  an  external  resistance  in  series  with 
the  armature,  and  the  other  objections  mentioned  as  regards  low 
efficiency  and  considerable  space  occupied  by  the  controller  also 
obtain.  Consequently,  this  method  should  be  employed  only  when 
the  periods  of  speed  adjustment  are  of  relatively  short  duration,  the 
motor  being  operated  most  of  the  time  at  rated  speed.  It  is,  however, 
used  considerably  in  connection  with  the  other  methods  of  speed 
control,  for  transition  from  one  running  speed  to  another. 

Speed  Control  by  Cascade  Connection.  — The  fourth  system  of 
induction  motor  speed  adjustment  is  variously  known  as  the  cascade, 
concatenation  or  tandem  control.*  The  application  of  this  method 
necessitates  the  use  of  at  least  two  motors,  the  revolving  members 
of  which  are  coupled  together,  either  directly  upon  the  same  shaft 
or  indirectly  by  the  load,  as  in  the  case  of  an  electric  locomotive. 
The  first  of  the  motors  (i.e.,  that  normally  connected  to  the  line) 
has  its  rotor  provided  with  a  polar  winding,  so  arranged  as  to  deliver 
at  standstill  a  voltage  of  the  same  pressure  and  number  of  phases 

*  C.  P.  Steinmetz,  Electrotechnische  Zeitschrift,  1899,  Vol.  XIX,  p.  884.  Speed 
Control  of  Induction  Motors,  H.  C.  Specht,  1909,  Elect.  Journal,  Vol.  VI,  Nos.  7  and  8. 
Multi-speed  Induction  Motors,  H.  Reist  and  H.  Maxwell,  Trans.  A.  I.  E.  E., 
Vol.  XXVIII,  1909,  p.  971.  Wechselstrom-technik,  E.  Arnold,  Vol.  V,  pp.  485-519, 
1909. 


208       ELECTRIC  MOTORS,    THEIR   ACTION  AND   CONTROL. 

as  is  furnished  by  the  power  circuit.  This  secondary  is  connected 
to  the  stator  winding  (primary)  of  the  second  motor.  The  rotor 
of  this  latter  machine  may  be  of  the  squirrel-cage  or  slip-ring  type. 
In  case  the  slip-ring  rotor  is  employed,  resistance  control  may  be 
utilized  for  transitional  steps. 

The  cascade  connection  of  two  three-phase  induction  motors  is 
shown  diagrammatically  in  Fig.  125.     Motor  A  receives  the  line 


PIG     125. TWO  THREE-PHASE  MOTORS  ARRANGED  FOR  CASCADE  OR 

INDEPENDENT  OPERATION. 
(Switch  S  up  and  S'  down  for  cascade  connection.) 


voltage  at  rated  frequency  upon  closing  the  supply  switch  S.  Its 
secondary  delivers  three-phase  currents  of  the  same  frequency  and 
voltage  to  the  stator  of  machine  B  when  jaw  of  switch  S'  is  con- 
nected to  its  lower  set  of  terminals;  consequently  both  motors  will 
accelerate.  However,  as  motor  A  speeds  up,  the  frequency  of  its 
rotor  currents  will  decrease,  and  at  fifty  per  cent  rated  speed  this 
latter  current  will  have  a  frequency  of  one-half  of  that  of  the 
line.  Motor  B  receiving  a  current  of  one-half  line  frequency 
will  also  run  at  one-half  speed.  Therefore,  if  both  motors  are 
coupled  as  above  shown,  this  half  speed  is  the  point  at  which 
the  machines  tend  to  operate  together.  Rated  speed  is  obtained 
when  one  machine  only  is  employed,  the  second  being  cut  out 
entirely  by  short-circuiting  the  slip-rings  of  rotor  A  by  means  of 
the  switch  S';  consequently  this  system  gives  a  two  to  one  speed 
adjustment. 

The  above  argument  applies  to  the  use  of  two  motors  having  an 
equal  number  of  poles.     If,  however,  the  machines  connected  in  cas- 


SPEED  CONTROL  OF  POLYPHASE  INDUCTION  MOTORS.      209 

cade  have  a  different  number  of  poles,  they  will  operate  at  speeds 
other  than  half  normal.  For  example,  referring  to  Fig.  125,  when 
either  motor  A  or  B  is  operated  singly,  the  synchronous  speed  in 
r.p.m.  =  frequency  X  60  -H  pairs  of  poles,  or  the  cascade  set  could 
be  employed  to  give  the  speed  of  either  motor,  depending  upon 
which  one  was  connected  to  the  line.  The  next  step  would  be  to 
connect  the  secondary  of  machine  A  to  that  of  the  primary  of  motor 
B,  short-circuiting  the  rotor  of  the  latter.  This  connection  also 
gives  one  of  two  speeds,  depending  upon  the  employment  of  direct 
or  differential  concatenation.  If  the  former  is  used,  both  motors 
tend  to  rotate  in  the  same  direction  and  the  synchronous  speed  of  such 
a  combination  is  given  by  the  expression: 

r.p.m.  =/X  60  +  (pA  +  pB)  (41) 

wherein /is  the  frequency  of  the  supply  circuit  in  cycles  per  second, 
while  pA  and  pB  are  the  pairs  of  poles  of  motors  A  and  B  respec- 
tively. 

Inverse  or  differential  concatenation  is  obtained  when  the  machines 
are  so  connected  that  they  tend  to  start  up  in  opposite  directions;  in 
such  case  the  synchronous  speed  is: 

r.p.m.  =  /  X  60  -5-  (pA  -  PB)-  (42) 

Cascade  connection  of  two  motors  having  a  different  number  of 
poles  consequently  provides  a  method  of  obtaining  a  four  speed 
outfit,  the  speed  range  depending  upon  the  number  of  poles  of  the 
respective  machines.  For  example,  if  motor  A  has  6  pairs  of  poles 
and  B  has  2  pairs,  while  the  line  has  a  frequency  of  60  cycles  per 
second,  the  following  synchronous  speeds  could  be  obtained : 

1.  Motor  B  operating  alone,   r.p.m.  =  /  X  60  -*•  pB  =  60  X 

60  -r-  2  =  1800. 

2.  Motors  A  and  B  connected    in    differential   concatenation, 

r.p.m.  =  /  X  60  -J-  (pA  ~  PB)  =  60  X  60  -*•  (6  -  2)  = 
900. 

3.  Motor  A  operating  alone,  r.p.m.  =  /  X  60  -*-  pA  =  60  X 

60  -r-  6  =  600. 

4.  Motors  A  and  B  connected  in  directed  concatenation,  r.p.m. 

=  /  X  60  -^  />A  +  />/?=  60  X  60  -^  (6  +  2)  =  450; 

or  a  speed  range  of  four  to  one  is  attained. 


210       ELECTRIC  MOTORS,    THEIR   ACTION  AND   CONTROL. 

The  torque  developed  by  a  group  of  motors  in  cascade  depends 
upon  whether  they  are  connected  in  direct  or  differential  order,  and 
it  may  be  determined  by  the  following  equation : 

Torque  in  Ibs.  at  i  ft.  radius  =  .117  (Wi  -  wt)  ^A  ^  *B  (43) 

wherein  Wi  represent  motor  watts  input,  a>L  watts  lost  in  primary 
of  the  motor,  pA  and  pB  number  of  pairs  of  poles  of  machines 
A  and  B  respectively,  and  /  the  frequency  of  the  supply  current  in 
cycles  per  second.  The  plus  sign  is  employed  in  case  of  direct 
concatenation  and  the  minus  sign  when  differential  connection 
is  used.  The  latter  gives  the  lowest  starting  torque,  and  the  set  will 
not  start  up  if  the  motor  having  the  larger  number  of  poles  is  con- 
nected to  the  line.  The  method  of  starting  in  such  instance  is  to 
speed  up  the  set  by  using  the  motor  with  the  smaller  number  of 
poles  singly,  and  when  the  synchronous  speed  for  differential  con- 
nection has  been  slightly  exceeded,  the  switches  are  thrown  over  so 
that  the  desired  differential  arrangement  is  secured,  after  which 
the  equipment  will  continue  to  work  properly.  It  is  possible  to 
operate  in  cascade,  having  the  motor  with  the  smaller  number  of 
poles  normally  connected  to  the  line,  which  condition  gives  a  self- 
starting  differential  arrangement,  but  such  order  of  connection  is 
not  particularly,  desirable,  because  the  iron  losses  of  the  set  would 
be  greatly  exaggerated,  owing  to  the  high  frequency  of  the  current 
in  the  secondary  circuit. 

The  characteristic  curves  of  a  group  of  induction  motors  con- 
nected in  cascade  can  be  determined  by  means  of  any  of  the  circle 
diagram  methods,  the  test  data  necessary  for  the  construction 
being  determined  in  substantially  the  same  manner  as  for  a  single 
machine.  The  power  factor  and  efficiency  of  a  cascade  group  of 
given  capacity  at  any  torque  and  speed,  on  account  of  the  combined 
wattless  components  and  losses,  will  be  lower  than  that  of  a  single 
machine  having  the  same  rating. 

A  great  advantage  of  the  two  motor  equipment  is  that  two  efficient 
running  speeds  can  be  obtained  without  opening  the  supply  switch, 
and  thus  the  possibility  of  producing  line  disturbances,  as  would 
occur  with  the  other  form  of  multi-speed  induction  motors,  is  elim- 
inated. 

It  is  possible  by  an  extension  of  the  cascade  connection  to  three 


SPEED  CONTROL  OF  POLYPHASE  INDUCTION  MOTORS.      211 


motors  to  obtain  a  very  wide  speed  range  of  many  steps.  For 
example,  by  using  on  a  6o-cycle  circuit  a  set  of  three  motors  having 
14,  8  and  2  pairs  of  poles  respectively,  a  speed  range  from  138  to 
1800  r.p.m.  is  secured.  The  cost  of  such  a  system  is,  however, 
extreme,  and  the  usual  demands  of  practice  are  more  economically 
met  by  employing  a  two  motor  set,  utilizing  gearing  to  secure  the 
wider  speed  ranges. 

Speed  Control  of  Variation  of  Applied  Potential.  —  The  slip  of  an 
induction  motor,  at  a  given  torque,  varies  approximately  inversely 
as  the  square  of  the  primary  voltage  (p.  182),  and  this  is  the  prin- 
ciple governing  the  action  of  the  potential  method  of  speed  control. 
The  usual  manner  of  securing  this  adjustable  voltage  is  through  a 
multi-tapped  compensator,  which  is  introduced  into  the  primary 
circuit.  The  connections  of  this  method  are  substantially  the  same 
as  those  of  the  compensator  starting  device  shown  in  Fig.  115  (p.  195), 
excepting  that  the  contactors  slide  over  the  taps,  instead  of  being 
fixed  in  position.  The  speeds  secured  at  different  values  of  poten- 


Rated 

75%  Rated  Tolrque 


50%  Rated  Torque 
25$  Kated  Torque 


60          70          80          90 
Percent  Eated  Volts 


110 


120        130        140        160 


FIG.  126. SPEED-VOLTAGE  CURVES  OF  A  THREE-PHASE  2O-H.P. 

INDUCTION  MOTOR. 

tial  with  various  values  of  torque  are  shown  in  the  curves  of  Fig.  126, 
and  from  these  the  speed  regulation,  for  any  selected  voltage,  upon 
change  in  torque  is  readily  obtained.  For  example,  the  speeds 
for  different  torques  at  50  per  cent  rated  potential  are  determined 
by  drawing  a  vertical  line  through  this  voltage  abscissa.  The 


212       ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

intersections  of  this  line  with  the  curves  gives  the  speed  developed 
at  the  corresponding  torques.  The  speed  regulation  of  a  motor  con- 
trolled by  this  method  is  very  poor,  while  the  power-factor  and 
efficiency  decrease  with  the  speed. 

The  regulation  and  efficiency  are  even  less  satisfactory  when 
adjustable  resistance  in  the  primary  is  employed  in  place  of  the 
compensator.  In  fact,  the  potential  method  of  induction  motor  speed 
control  is  most  unsatisfactory.  Its  use  should  not  be  at  all  recom- 
mended except  perhaps  in  the  special  case  of  traveling  cranes, 
where  the  more  desirable  methods  already  considered  introduce  too 
great  a  complication  in  wiring  and  trolley  connections. 

For  further  information  concerning  induction  motor  speed  control  see  the  following: 
ALTERNATING  CURRENTS.    A.  Hay.    London,  1906. 

DIE  REGULIRUNG  VON  DREHSTROM-MOTERN.  W.  Burkard.  Elektrotech-Zeit, 
August,  1903. 

DIE  TOURENREGULIRUNG  VON  INDUKTIONS  MOTERN.    M.  Osnos.    Elektrotech-Zeit, 

1902,  p.  1075. 

SPEED  CONTROL  BY  FREQUENCY  CHANGERS.  H.  C.  Specht.  Elect.  Journal,  Vol.  VI, 
1909. 

SPEED  CONTROL:  POLYPHASE  MOTOR.  B.  G.  Lamme.  Electric  Journal,  Vol.  I, 
1904;  Vol.  VI,  1909. 

THREE -PHASE  MOTORS  —  WIDE  SPEED  RANGE.  Dr.  H.  B.  Eschenburg.  Electri- 
cian, London,  1903. 

TOURENREGULIRUNG  VON  INDUKTIONS  MOTERN.  J.  K.  Sumac.  Zeit-fur  Elek.-Tech., 
1904. 

WECHSELSTROMTECHNIK.    Vol.  V.    E.  Arnold.     1909. 


CHAPTER   XVII. 

THE   SINGLE-PHASE   INDUCTION   MOTOR.* 

THE  simplicity  of  single-phase  systems  in  comparison  with  poly- 
phase ones  makes  them  more  desirable  for  small  alternating-current 
plants.  The  constant-speed  motor  most  extensively  used  in  con- 
nection with  such  service  is  of  the  single -phase  induction  type  and 
structurally  it  is  very  similar  to  the  corresponding  polyphase 
machine.!  In  fact  any  polyphase  induction  motor  will  operate 
as  a  single-phase  machine  of  somewhat  smaller  capacity  and  lower 
power  factor,  if  it  is  at  first  caused  to  rotate  at  nearly  synchronous 
speed  by  some  starting  arrangement.  The  necessity  of  providing 
some  such  auxiliary  device  arises  from  the  fact  that  the  single-phase 
motor,  per  se,  has  no  starting  torque.  That  such  is  the  case  may 
be  readily  seen  without  the  introduction  of  mathematical  proof. 

Absence  of  Starting  Torque.  — Consider  a  bipolar  single-phase 
motor,  provided  with  a  squirrel-cage  rotor.  The  distribution  of 
current  in  the  secondary  at  standstill  is  as  indicated  in  Fig.  127. 
The  current  in  bars  aa'  is  zero,  because  these  are  equivalent 
to  a  closed  loop  the  plane  of  which  is  parallel  to  the  flux.  The 
maximum  current  is  set  up  in  bars  bb> '.  However,  this  equivalent 
loop,  if  it  moves  at  all,  must  move  parallel  to  the  direction  of  the 
lines  of  force,  hence  it  exerts  no  turning  effort.  The  bar  m,  carry- 
ing current  as  indicated,  will  exert  a  torque  upon  the  rotor,  as 
shown  by  the  arrow  alongside  it.  However,  owing  to  the  symmetry 
of  the  secondary  winding,  for  every  bar  m  there  is  another  m'  hav- 
ing a  current  of  equal  amplitude  but  of  opposite  sign.  This  latter 
bar  being  in  a  field  of  the  same  strength  and  direction  as  that  in 
which  m  is  located,  will  exert  a  torque  equal  to  that  developed  by 
m,  but  in  the  reverse  direction,  as  indicated  by  the  corresponding 
arrow.  In  the  same  way  the  effort  exerted  due  to  the  current  in 

*  The  Single-phase  Induction  Motor,  J.  H.  Morecroft  and  M.  Arendt,  G.  E. 
Review,  Vol.  13,  No.  5,  1910. 

t  The  first  successful  motor  of  this  type  was  built  by  C.  E.  L.  Brown,  see  London 
Electrician,  Vol.  XXX,  p.  358,  1893. 

213 


214        ELECTRIC  MOTORS,    THEIR  ACTION   AND    CONTROL. 

any  bar  of  the  winding  will  be  neutralized  by  that  of  another  sym- 
metrically located  with  respect  to  the  axis  of  the  primary  field, 
consequently  at  standstill  no  turning  effort  is  developed  and  the 
motor  fails  to  accelerate. 

b' 


FIG.   I2y. — DISTRIBUTION  OF  CURRENT  IN  STATIONARY  ROTOR  OP 
SINGLE-PHASE  INDUCTION  MOTOR. 

The  above  fact  may  be  proved  as  follows:  Assume  the  rotor 
winding  as  composed  of  symmetrically  placed  short-circuited  coils, 
and  consider  one  having  its  plane  at  any  angle  a  to  the  axis  of  the 


FIG.    128. — SHORT-CIRCUITED  COIL  INCLINED  TO  AXIS  OF  MAGNETIC  FIELD. 

field  NS,  as  illustrated  in  Fig.  128.  Further  suppose  the  flux 
distribution  to  be  a  cosine  function  of  a;  this  is  approximately  the 
case  with  actual  motors  provided  with  distributed  stator  windings, 


THE  SINGLE-PHASE   INDUCTION  MOTOR.  215 

and  then  let 

B  represent  the  maximum  flux  density  at  a,  —  o°, 

B  cos  pt  represents  the  instantaneous  flux  density  at  a,  =  o°, 

B  cos  pt  cos  a  represents  the  corresponding  value  at  the  inductors 

selected,  and  with  A  as  the  area  of  the  coil  the  flux  passing  through 

it  becomes 

/a 
AB  cos  pt  cos  ada  =  BA  cos  pt  sin  a.  (44) 

„ 

The  e.m.f.  induced  in  the  selected  coil  is 

d$ 

e  = 7-  =  BAp  sin  pt  sin  a.  (45) 

at 

The  instantaneous  value  of  the  corresponding  current  is 

i  =  BAp  sin  (pt  —  6)  sin  a  -*•  Z1 '.  (46) 

Naturally  in  the  case  of  a  single  coil  this  current  will  react  upon 
the  stator  field  and  produce  flux  distortion ;  but  as  we  are  going  to 
sum  up  the  effects  of  all  the  rotor  coils,  the  individual  reactions 
balance,  and  the  field  distortion  becomes  negligible.  It  is  to 
be  noted  that  the  impedance  of  a  coil  will  be  modified  by  the 
action  of  the  neighboring  coils,  consequently  Z'  in  equa.  46  repre- 
sents the  effective  impedance.  The  angle  0  =  cos"1  ( rf  -±  Z'),  where- 
in /  is  the  effective  resistance  of  the  coil  and  Z'  the  impedance 
as  above  defined. 

If  there  are  n  coils  on  the  rotor  equally  spaced  from  one 
another,  the  torque  of  the  Kth  coil  will  be 

tk  =  lB*Ap  [sin  (2  pt  -  6)  +  sin  6] sin  —  n  H-  2  Z',  (47) 

n 

wherein  I  is  the  length  of  one  coil. 

The  instantaneous  torque  exerted  by  the  whole  rotor  is 

T=  S/  =  lB2Ap[sm(2pt  -6)+  sin  0]Stnsin—  TT  +  2Z'=o* 

n 

(48) 

Development  of  Revolving  Field.  —  We  have  just  shown  that 
when  we  have  an  oscillating  magnetic  field  the  rotor  placed  therein 
fails  to  exert  any  starting  torque.  Therefore,  if  a  single-phase 
induction  motor  does  develop  a  turning  effort  after  it  is  caused 
to  revolve,  it  must  be  because  it  has,  by  some  reactions  of  the  rotor 

*  This  same  result  is  obtained  from  analysis  of  equation  60,  p.  225. 


216        ELECTRIC  MOTORS,    THEIR  ACTION   AND    CONTROL. 

currents  upon  the  stator  flux,  provided  for  itself  a  rotating  magnetic 
field.  That  such  is  the  case  may  be  shown  non-mathematically. 
Assume  a  two-pole  motor  (Fig.  129)  the  stator  winding  of  which  is 
supplied  with  a  single-phase  alternating  current,  producing  an 
oscillating  field  between  the  poles  A  A '.  The  rotor  currents  pro- 


FIG.   129.  —  MAIN  AND  QUADRATURE  FIELDS,  SINGLE-PHASE  INDUCTION  MOTOR. 

duce  a  field  at  right  angles  to  the  main  field,  and  for  convenience 
we  will  assume  this  to  be  represented  by  the  poles  BB'.  In  com- 
mercial machines  no  such  empty  pole  spaces  exist,  as  practically  all 
of  the  stator  is  covered  with  coils. 

The  inductors  of  the  revolving  rotor  have  e.m.f  s  induced  in 
them  due  to  two  actions,  namely  by  motion  through  the  field  and 
by  the  time  rate  of  change  of  the  flux  threading  the  coils.  The 
first  we  shall  designate  as  a  rotational  e.m.f.  and  the  second  as  a 
transformer  e.m.f. 

The  inductors  aa'  will  always  have  a  rotational  e.m.f.  set  up  in 
them  except  when  the  stator  field  passes  through  zero  value.  The 
amplitude  of  this  e.m.f.  for  any  given  speed  will  be  proportional 
to  the  instantaneous  value  of  the  stator  flux.  Conductors  aa'  may 
be  considered  equivalent  to  closed  coils,  and  the  current  flowing 
in  them  will  produce  a  field  in  direction  BB'.  Neglecting  tem- 
porarily the  IR  drop  in  the  rotor,  the  e.m.f.  induced  in  aa'  may  be 

placed  equal  to  -,  where  3>r  denotes  the  cross  field  developed  by 


THE  SINGLE-PHASE   INDUCTION   MOTOR.  217 

the  currents  due  to  the  motion  of  the  rotor  in  the  main  field.  The 
rotational  e.m.f.  is  in  time  phase  with  the  main  field,  hence  the  cross 
field  <l>r  will  be  in  time  quadrature  with  it.  The  direction  of  the  main 
field  and  the  motion  of  the  rotor  inductors  are  such  that  the  e.m.f. 
generated  in  aa'  is  positive.*  The  rotor  currents  are  in  such 
direction  that  when  pole  A  is  of  north  polarity  and  decreasing, 
pole  B  will  be  of  like  sign  but  increasing,  reaching  its  maximum 
strength  one-quarter  of  a  period  later.  The  strength  of  pole  B 
decreases  after  a  similar  lapse  of  time,  the  main  field  reverses  and  a 
north  pole  begins  to  build  up  at  A'.  That  is,  the  main  field  and 
quadrature  field  so  combine  that  a  north  pole  travels  around  the  stator 
in  the  direction  ABA'B'  at  synchronous  speed.  Hence  there  exists 
a  rotating  field  produced  by  the  combined  action  of  stator  and 
rotor  currents.  This  simple  explanation  gives  an  idea  of  the  pro- 
duction of  the  rotating  field  in  the  single-phase  induction  motor,  but 
it  does  not  consider  all  the  reactions  which  occur. 

The  inductors  W  moving  in  the  quadrature  field  have  a  rota- 
tional e.m.f.  induced  in  them,  in  the  same  manner  as  those  passing 
through  the  main  field,  and  this  is  of  maximum  positive  value 
when  the  north  pole  at  B  attains  its  highest  value.  In  addition  to 
these  two  rotational  e.m.f.'s,  the  varying  fields  A  A'  and  BB'  set  up 
transformer  e.m.f 's,  in  coil  groups  bbf  and  aaf  respectively.  Conse- 
quently, there  are  four  e.m.f. 's,  to  be  considered  before  the  actual 
rotor  currents  which  produce  the  quadrature  field  can  be  determined. 

The  rotational  e.m.f.  induced  in  inductors  aa'  is  of  maximum 
positive  value  when  the  pole  A  is  at  its  greatest  north  polarity,  but 
the  transformer  e.m.f.  set  up  in  these  bars  by  the  quadrature  field 
is  at  the  same  moment  of  maximum  negative  value.  Hence  the 
actual  e.m.f.  (Ea)  existing  in  A  A '  is  the  algebraic  sum  of  these  two 
voltages.  The  rotational  e.m.f.  due  to  the  main  field  must  be 
greater  than  the  transformer  e.m.f.  of  the  quadrature  field;  in  fact 
the  latter  is  of  such  strength  that  the  actual  e.m.f.  (Ea)  will  be 
just  enough  to  establish  the  current  which  produces  the  field  BB'. 
Since  this  quadrature  field  is  at  right  angles  to  the  main  field,  its 
m.m.f.  cannot  be  furnished  directly  by  the  stator  magnetizing 
current,  so  we  must  investigate  further  to  see  how  it  is  taken,  as  it 
must  be,  from  the  line.  It  must  be  remembered  that  the  imped- 

*  Currents  flowing  away  from  the  reader  into  the  plane  of  the  paper  are 
called  positive. 


218        ELECTRIC   MOTORS,    THEIR  ACTION   AND   CONTROL. 


ance  of  the  rotor  coils  is  here  assumed  to  be  such  that  the  IZ  drop 
is  negligible;  if  this  is  not  the  case,  the  rotational  and  transformer 
e.m.f.'s  will  not  be  in  time  opposition  and  then  vector  sum,  instead 
of  algebraic  sum,  must  be  considered. 

The  main  field,  by  transformer  action,  induces  an  e.m.f.  in  bars 
bb',  and  this  is  opposed  to  the  e.m.f.  developed  in  the  same  inductors 
by  their  motion  through  the  quadrature  field.  The  resultant  e.m.f. 
(Eb]  in  these  conductors  sets  up  a  current  affecting  the  main  field 
and  consequently  the  current  drawn  from  the  line.  The  current 
flowing  in  inductors  W  due  to  Eb  is  equal  to  that  existing  in  bars 
aa',  which  latter  is  that  producing  the  cross  m.m.f.  Moreover,  the 
current  W  is  in  such  direction  that  it  increases  the  magnetizing 
current  taken  from  the  line,  the  increment  being  that  which  would 
be  necessary  to  directly  magnetize  the  quadrature  field.  The 
reluctance  of  the  cross  field's  magnetic  circuit  is  substantially  the 
same  as  that  of  the  main  field,  consequently  the  m.m.f.  required 
for  both  will  be  the  same,  and  obviously,  therefore,  a  two-phase 

motor  run  on  one  phase 
will  draw  twice  its  nor- 
mal magnetizing  current. 
This  conclusion"  is  borne 
out  by  actual  practice, 
tests  showing  that  the 
magnetizing  current  of  a 
single  -  phase  motor  is 
double  that  taken  per  phase 
by  a  two-phase  and  three 
times  that  required  by  a 
three-phase  machine,  the 
potential  difference,  fre- 
quency and  turns  per  phase 
winding  being  the  same. 

At  synchronous  speeds 
the  two  component  fields 
are  of  equal  strength; 
accordingly  they  com- 
bine to  give  a  circularly 
rotating  field.  Below  synchronous  speed  the  rotating  e.m.f.  in  the 
bars  aaf  is  reduced  in  inverse  proportion  to  the  slip,  and  thus  the 


FIG.    130.      FORMS  OF  ROTATING  FIELD 
AT  VARIOUS  ROTOR  SPEEDS. 


THE  SINGLE-PHASE  INDUCTION  MOTOR. 


219 


quadrature  field  diminishes,  while  the  main  field  remains  constant. 
Consequently  the  rotating  field  developed  below  synchronous  speed 
is  of  an  elliptical  form,  the  shorter  axis  being  in  the  direction  of  the 
quadrature  field  BB'.  When  driven  above  synchronous  speed 
the  field  is  also  of  elliptical  form,  the  major  axis,  however,  being  in 
the  direction  of  the  cross  field.  The  field  forms  for  different  speeds 
are  as  illustrated  in  Fig.  130,  a,  b,  c,  respectively,  corresponding  to 
synchronous,  sub-synchronous  and  super-synchronous  speeds. 

The  maximum  torque  which  a  motor  is  capable  of  exerting,  other 
things  being  equal,  depends  upon  the  average  value  of  the  magnetic 
field  in  which  the  rotor  moves.  This  mean  value,  neglecting  IR 
drop  and  leakage,  is  in  the  polyphase  induction  motor  independent 
of  the  slip,  while  for  the  corresponding  single-phase  machine  the 
average  value  of  the  field  decreases  as  the  slip  increases;  thus  the 
pull-out  torque  of  a  polyphase  machine  connected  single  phase  will 
be  less  than  when  normally  operated. 

Many  interesting  facts  concerning  the  rotor  currents  as  well 
as  the  development  of  the  rotating  field  may  be  derived  through 


FIG.    131.      COILS  INCLINED  TO  AXIS  OF  OSCILLATING  FIELD. 


a  simple  mathematical  analysis.  Let  us  consider  the  elementary  bi- 
polar single-phase  induction  motor  represented  in  Fig.  131  with  a 
coil  at  an  angle  a  to  the  main  polar  axis.  Assume  as  before  (p.  214) 
that  the  flux  distribution  is  a  cosine  function  of  time,  and  adopt 
the  following  notation: 


220        ELECTRIC  MOTORS,    THEIR   ACTION   AND   CONTROL. 

A  =  area  of  coil. 

cu  =  angular  velocity  of  the  coil,  or  a  =  ajt. 
A  sin  a  =  A  sin  a>t  =  projected  area  of   coil  on  plane  CC' 
perpendicular  to  the  flux  NS. 

B  =  maximum   flux    density,  its    instantaneous   value   being 
B  cos  pi. 

Instantaneous  flux  interlinking  coil  a  is 
3>  =  AB  cos  pt  sin  wt 

=  .5  AB  [sin  (p  +  aj)  t  -  sin  (p  -  a>)  /];  (49) 

the  e.m.f.  induced  in  coil  a  is 


e=  --T-  =  .$AB  {(p-<o)cos(p-<u)t-(p+a>)cos 

(5°) 

Let  rt  and  Zt  represent  respectively  the  effective  resistance  and 
inductance  of  the  coils;  the  values  of  these  constants  are  based  not 
only  upon  the  character  of  an  individual  coil  but  also  to  some  extent 
upon  -the  action  of  neighboring  coils.  With  this  notation  the  cur- 
rent in  any  secondary  coil  can  be  considered  as  resulting  from  the 
e.m.f.  of  equa.  50,  or 

p  —  a) 


V+tt-aO'A']* 


cos  [(#-a>) /-<?,] 


wherein 


'•~°*~\rl'+(P+»yLlf 


The  fluxes  produced  by  one  rotor  coil  and  the  main  field  will  so  react 
upon  each  other  that  the  value  of  the  secondary  current,  if  but  a 
single  coil  be  considered,  can  be  expressed  only  by  an  infinite 
series.  It  has  been  experimentally  shown,  however,  that  the 
flux-distorting  reactions  between  primary  and  secondary  do  not 
exist  with  a  rotor  winding  composed  of  a  number  of  coils  which 
are  divisible  into  pairs,  the  members  of  which  are  placed  at  90 
degrees  (electrical)  to  each  other.  The  rotor  winding  of  a  commer- 
cial machine  substantially  satisfies  this  condition;  consequently  the 


THE  SINGLE-PHASE  INDUCTION  MOTOR.  221 

higher  harmonics  of  the  rotor  current  disappear  and  the  same  is 
correctly  represented  byequa.  51  given  above.  This  equation  indi- 
cates that  the  rotor  current  consists  of  two  parts  having  different  fre- 
quencies and  amplitudes. 

At  standstill  any  coil  spaced  an  angle  y  from  the  axis  of  the  mag- 
netic field  will  have  a  current  of  the  following  form: 


which  shows  that  the  secondary  current  at  standstill  is  of  line 
frequency.  The  current  component  with  frequency  (p  —  co)  de- 
creases in  value  as  the  rotor  speed  rises  toward  synchronism,  being 
zero  at  that  limit,  and  the  secondary  current  then  becomes 


,  _  , 

'2 


which  is  of  double-line  frequency. 

These  variations  of  rotor  current  frequencies  as  well  as  the  pres- 
ence of  the  differential  (p  —  CD)  and  additive  (p  +  aj)  compo- 
nents may  be  conveniently  observed  by  the  application  of  a  reed 
frequency  meter.  Connect  such  an  instrument  across  the  slip  rings 
of  the  wound  rotor  of  a  polyphase  motor,  excite  the  stator  with 
single-phase  current  and  then  start  the  machine.  As  the  speed 
of  the  rotor  increases,  the  frequency  meter  will  indicate  the  presence 
of  two  currents,  one  increasing  and  the  other  diminishing  from 
the  line  frequency. 

Let  us  now  select  a  coil  on  the  rotor  displaced  any  angle  /?  from  the 
loop  a  we  have  just  considered,  Fig.  131.  The  flux  through  this  new 
coil  at  synchronous  speed  (a  =  cot  =  pt)  will  be,  from  equa.  (49), 

.  :   $  =  AB  cos  pt  sin  (pt  +  /?), 

(54) 


e.m.f.  coil  /?  =  e  =  -  —  =  -ABp  (cos  (2  pt  +  /?),  (55) 

current  coil  /?  =  *  =  --          ^       cos  (2  pt  +  p  —  6),  (56) 
v  r  2  +  2  pL? 

-  KlCa*  (2  pt  +  ft  -  0).  (57) 


222      ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

The  total  magneto-motive  force  of  all  the  coils  on  the  rotor  may  be 
expressed  as  K^i.  The  maximum  m.m.f.  exists  in  the  plane  of 
the  coil  in  which  the  current  is  equal  to  zero,  and  hence  the  poles  of 
the  rotor  will  be  in  the  same  plane.  Let  p/  be  the  angle  of  that 
particular  coil;  then 

*  =  K^cos  (2pt  +  /?'-  6). 
But  since  i  is  equal  to  zero, 

£,  cos  (2  #/  +  /?-  0)=o, 
whence 

7£ 

2  pt  ~\~  p   —   0  =  ~ 
2 

and 

/~       \ 

-  2pt. 

This  means  that  the  angle  between  the  reference  coil  and  the 
magnetic  pole  of  the  rotor  changes  at  the  rate  of  —  2pt.  It  also 
indicates  that  the  pole  rotates  backwards  on  the  rotor.  The  latter, 
however,  is  turning  forward  at  a  rate  pt,  consequently  the  rotor 
poles  revolve  backward  in  space  at  a  rate  pt,  and  the  equation  of 
this  pole  in  space  is 


If  the  equation  for  the  current  in  the  general  coil  is  referred  to  the 
magnetic  axis  instead  of  to  the  reference  coil,  we  have 


*  =  KlCos\(2  pt  +  p-  $)+(-  +  0- 

\2 


That  is,  referred  to  the  magnetic  axis  of  the  rotor  the  current 
distribution  is  constant,  hence  the  m.m.f.  of  these  currents  is 
constant  and  rotates  backward  at  synchronous  speed,  as  above 
proved. 

The  relative  value  of  the  stator  and  rotor  m.m.f  s  may  be  derived 
as  follows:  Assume  the  rotor  stationary;  this  corresponds  to 
considering  it  the  same  as  the  short-circuited  secondary  of  a  trans- 
former. Thus  the  relations  existing  between  primary  and  secondary 
m.m.f  's  of  a  transformer  apply  or,  neglecting  resistance  and 
leakage,  the  secondary  m.m.f.  is  equal  and  opposite  to  that  of  the 


THE  SINGLE-PHASE  INDUCTION  MOTOR.  223 

primary.     The  current  distribution  in  the  bars  on   the    rotor  on 
the  basis  of  the  above  assumption  is  expressed  by  equa.  46  as 

sin  (pt  -  6) sin/?, 


Vr2+' 

£  

which  upon  neglecting  r  makes  0  =  -  and  reduces  to 

2 


V=-— : ^  cos  ^  sin/?; 
r*« 

this  if  /  =  o  becomes 

*0=-4^sin/?.  (58) 

o  T 

It  is  to  be  noticed  that  when  /  =  o,  the  equation  of  the  rotor  cur- 
rents at  synchronous  speed  (equation  56)  reduces  to 


which  can  be  still  further  simplified,  if  r  is  negligibly  small  with 
respect  to  pL,  to  the  following  form, 

jr=-  — sin/?.  (59) 

Comparing  these  values  of  i0  and  ir  we  see  that  these  currents  have 
the  same  distribution  in  the  rotor,  but  the  amplitude  of  the  latter 
is  only  one-half  that  of  the  former.  Consequently,  since  the  m.m.f's 
of  the  stationary  rotor  and  of  the  stator  are  equal,  the  m.m.f.  of 
the  synchronously  revolving  rotor  is  one -half  that  of  the  stator 
winding. 

The  magneto-motive  force  effective  in  developing  the  flux  B  cos  pt 
when  the  two  fields  coincide  may  be  expressed  as  F  —  X,  wherein 
Y  represents  the  maximum  m.m.f.  developed  by  the  stator  and  X 
that  due  to  the  rotor.  But,  as  above  shown,  X  =  Y  +  2,  hence 
the  excitation  necessary  to  produce  the  flux  B  cos  pt  throughout  the 

Y 

magnetic  circuit  of  the  machine  is  —  or  -X". 

The  two  magneto-motive  forces  acting  at  any  instant  in  this 
type  of  machine  are: 

Y  cos  pt,  stationary  in  space. 

X,  constant  in  value,  but  rotating   backward   at   synchronous 


224        ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

speed.  Since  X  rotates  backwards  it  may  be  written  X  =  X  cos  pi 
—X  sin  pt,  and  consequently  Y  —  X,  the  total  magneto-motive  force 
acting  at  any  instant,  becomes 

Y  cos  pt  -  X  cos  pt  +  X  sin  pt  =  X  cos  pt  +  X  sin  pt. 

This  means  that  the  total  m.m.f.  acting  at  any  instant  is  of  constant 
value  and  rotates  forward  at  synchronous  speed. 

The  magnetic  reluctance  of  commercial  single-phase  motors,  due 
to  the  use  of  uniformly  distributed  windings,  is  practically  the 
same,  whatever  the  axis  of  the  field,  consequently  the  reactions 
existing  between  stator  and  rotor  currents  produce  at  or  near 
synchronous  speed  a  circular  rotating  field,  and  the  formulae 
which  apply  to  polyphase  motors  may  be  utilized.  The  effect  of 
leakage  and  rotor  resistance  will  modify  this  rotating  field  somewhat, 
changing  it  from  circular  to  elliptical  form. 

Torque  Equations.  —  It  has  been  indicated  on  p.  220  that  when 
the  secondary  of  a  single-phase  induction  motor  is  caused  to  rotate 
at  any  rate  a>,  its  current  may  be  expressed  as 


cos 


Inspection  of  this  equation  shows  that  the  rotor  current  is  composed 
of  two  parts,  one  of  a  lower  and  the  other  of  a  higher  frequency 
than  the  rotating  field.  We  may  consequently  consider  that  this 
current  is  set  up  through  the  action  of  two  synchronously  rotating 
fields,  one  revolving  in  the  same  direction  as  the  rotor  and  the  other 
oppositely.*  The  frequency  of  the  rotor  current  component  due 
to  the  suppositional  field  revolving  in  the  same  direction  as  the 
rotor  is  naturally  less  (by  the  velocity  of  the  rotor)  than  synchronous 
value  or  it  is  (p  —  a>).  The  component  due  to  the  oppositely 
rotating  field  has  a  frequency  higher  than  that  of  the  line,  its  value 
being  (p  -f  <w). 

The  per  cent  slip  of  the  rotor  with  respect  to  the  first  field  is 

I- )  100,  and  referred  to  the  second  field  it  is  I- ) 

\     p     /  \     P     / 

*  G.  Ferraris,  Mem.  Reale  Accad.  di  Scienze  Torino,  Series  II,  Vol.  xliv,  December 
1893.     Electrician,  Vol.  33,  pp.  no,  129,  152,  184.     London,  1894. 


100. 


THE  SINGLE-PHASE  INDUCTION  MOTOR.  225 

The  effective  turning  effort  of  the  motor  is  the  resultant  of  the 
interaction  between  the  rotor  current  and  two  oppositely  rotating 
fields.  But,  since*the  rotor  and  one  field  turn  in  the  same  direction, 
the  torque  due  to  this  latter  field  must  be  greater  than  that  set  up 
by  the  other.  We  have  seen  from  equa.  38  (p.  180)  that  the 
torque  developed  by  a  polyphase  induction  motor  is  expressed  by  the 
following  equation  : 


wherein  s  is  the  per  cent  slip  between  rotating  field  and  rotor  core, 
while  co^  =  p  is  the  angular  velocity  of  the  revolving  field.  We  may 
accordingly  write  the  two  component  torques  existing  in  the  single- 
phase  motor  as 


1 


_      _ 

n  ~~ 


wherein  $ 


^(rS  +  sJXS)' 

+  (*)         0).  +  0) 


The  total  effective  torque  is 


T  -  T  +  T  -       ***-  a2-» 

r>+        -kfr.^V^WH1^)1 

wherein  s2  —  sl  is  positive  for  speeds  below  synchronism,  while  s^ 
is  variable  but  never  greater  than  unity. 

Analysis  of  this  equation  brings  out  the  following  facts: 

1.  That  the  torque  of   the   single-phase  machine  varies  as  the 
square  of   the  impressed  voltage,  this  being  the  same  relation  as 
obtains  in  polyphase  induction  motors. 

2.  That  the  motor  exerts  no  torque  at  standstill  because  s2—  s1 
then  equals  zero,  which  makes  the  numerator  of  the  same  value.* 

3.  The  motor  cannot  operate  at  synchronous  speed,  because  this 
makes  st  zero,  in  which  case  the  torque  developed  is  of  negative 
value,  s^s^X^—r*  reducing  to  —  r22,  and  the  machine  tends  to  act  as 
a  generator.     Consequently  the  single-phase  induction  motor  must 
rotate  at  less  than  synchronous  speed. 

*  (See  equations  (47)  and  (48),  p.  215.) 


226        ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 


4.  The  maximum  value  of  sls2  being  unity  indicates  that  the 
single-phase  induction  motor  cannot  operate  unless  the  reactance 
of  its  rotor  winding  at  standstill  is  greater  than  its  resistance.  Un- 


less  such   is    the   case 


—  r 


will   have  a  negative   value, 


which  means  that  the  machine  would  tend  to  develop  a  negative 
100 


75 


50 


25—0+25         50         75 
Percent  Rated  Torque 


100 


125        150 


FIG.    132. SPEED-TORQUE   CURVES   WITH   ROTOR   RESISTANCE   VARIED. 

torque  or  act  as  a  generator.  Fig.  132  indicates  how  the  speed- 
torque  curves  of  a  single-phase  induction  motor  are  affected  by 
change  in  the  value  of  rotor  resistances.  Curves  A  and  B  may  be 
considered  as  representative  of  standard  machines.  Curves  C  and 
D  indicate  the  effects  produced  by  inserting  relatively  large  resist- 
ances into  the  rotor  winding.  It  is  apparent  from  these  curves 
that  the  introduction  of  resistance  into  the  rotor  circuit  for  purposes 
of  speed  regulation  is  attended  by  a  marked  reduction  of  the  over- 
load capacity  of  the  motor,  and  cannot  be  used  as  conveniently 
or  advantageously  as  with  polyphase  motors  (p.  201).  It  is, 
however,  employed  to  limit  the  starting  current  (pp.  199  and  238). 

5.  The  torque  developed  by  a  polyphase  motor  operated   as   a 
single-phase  machine  is  less    than    that  produced  when  normally 
connected,  because  of  the  presence  of  the  counter  torque  T2. 

6.  If  we  take  the  first  differential  coefficient  of  equa.  60  with 
respect  to  r2  and  place  it  equal  to  zero,  we  find  that  the  maximum 
torque  developed  for  any  rotor  speed  a>  exists  when 

Y  —  Y  TV  *  -*-  (TV*  -i-  ? } 

ri —  -^r^i^a        v<?i*a      '    */> 


THE  SINGLE-PHASE  INDUCTION  MOTOR.  227 

and  that  the  maximum  torque 


This  equation  shows  that  the  torque  at  any  selected  speed  is  greater 
the  less  the  value  of  ry 

Characteristic  Curves.  —  The  preceding  torque  equation,  while 
valuable  in  that  it  indicates  the  general  characteristics  of  single- 
phase  induction  motors,  is  not  readily  applied  to  the  detail  study  of 
any  specific  machine.  The  working  curves  are  most  accurately 
determined  by  actual  test.  They  may,  however,  be  derived  with 
moderate  accuracy  by  means  of  a  circle  diagram  somewhat  similar 
to  that  utilized  in  connection  with  the  study  of  the  polyphase  motor. 

The  particular  diagram  described  herein  (Fig.  133)  is  that  de- 
veloped by  A.  S.  McAllister,  and  its  construction  is  as  follows:* 

Let  the  vertical  line  OE  (Fig.  133)  represent  the  line  voltage. 
Draw  at  their  proper  phase  positions  and  scale  values  the  no-load  as 
well  as  the  locked  currents  OM  and  OF  respectively.  The  value  of 
OF  is  determined  as  already  explained  in  connection  with  the 
polyphase  induction  motor  (p.  186). 

MN  and  IF  represent  the  energy  components  of  the  corre- 
sponding currents  and  are  therefore  directly  proportional  to  the 
respective  inputs.  Through  M  draw  a  line  MK  perpendicular  to 
OE,  join  M  and  F;  draw  also  a  line  perpendicular  to  the  middle  of 
MF  intersecting  MK  at  X.  With  X  as  a  center  and  either  XM 
or  XF  as  a  radius,  describe  the  circular  arc  MPF,  this  being  the 
locus  of  the  primary  current.  The  distance  IG  represents  the 
added  primary  or  stator  loss  existing  with  the  rotor  locked,  its 
length  =  (added  primary  copper  loss  -J-  total  locked  watts)  X  HF. 
Draw  the  line  GM.  With  this  construction  completed  the  perform- 
ance of  the  motor  may  be  determined  by  inspection.  For  example, 
the  factors  determining  the  performance  of  the  motor  with  a  current 
P  are  as  follows: 

OP  to  scale  represents  the  primary  current. 
cos  POE  is  the  power  factor  of  current  OP. 
PT  represents  the  watts  input  at  current  OP. 
MN  represents  the  watts  input  at  no  load. 
TQ  represents  the  watts  loss  at  current  OP. 

*  Alternating  Current  Motors,  A.  S.  McAllister,  pp.  115-119.  McGraw  Co., 
New  York  1909. 


228        ELECTRIC  MOTORS,   THEIR  ACTION  AND   CONTROL. 


Percent  Power  Factor 
0    10    20    30    40    50    60    70    80    90    100 


V\     Scale  :- 

1  cm.  =  10  Amperes. 

1  cm.=2.2K.W. 

\\ 


10 


133. — MCALLISTER  CIRCLE  DIAGRAM  FOR  A  IO-H.P.  SINGLE-PHASE 
INDUCTION  MOTOR. 


THE  SINGLE-PHASE  INDUCTION   MOTOR. 


229 


TR  represents  the  total  primary  loss  at  current  OP. 
QR  represents  the  added  secondary  copper  loss. 
QP  represents  the  watts  output  at  current  OP. 
QP  -T-  PT  represents  the  efficiency  of  the  motor  at  current  OP. 
ico  (PQ  +  PR)*  represents  the  per  cent  slip. 
7.05  QP  -v-  r.p.m.  represents  the  torque  at  current  OP. 
The  field  set  up  through    he  motion  of  the  rotor  varies  as  the 
speed  (a),  consequently  the  torque  (T)  for  a  given  rotor  input  (W) 
will  be  proportional  to  the  product  of  toW,  or 

T  =  K^W.  (62) 

The  torque,  however,  is  also  proportional  to  the  secondary  output 
(W")  divided  by  the  speed  (at),  or 

T  =  K2  (W"  •*•  to) 
whence  w  =  K  (W"  +  W')*  (6.3) 


5      10     15     20    25     30    35     40    45     50 
Ft.  Lbs.  Torque 

FIG.   134.  —  CHARACTERISTIC  CURVES  OF  A  22O-VOLT,  6O-CYCLE,   4-POLE, 
IO-H.P.   SINGLE -PHASE   INDUCTION  MOTOR. 

The  secondary  input,  from  the  circle  diagram,  is  proportional 
to  PR',  the  output  is  similarly  represented  by  PQ-,  consequently 
(PQ  +  PR)*  corresponds  to  the  rotor  speed  as  above  stated. 

The  diagram  shown  in  Fig.  133  has  been  applied  to  the  deter- 
mination of  the  characteristic  curves  of  a  standard  2 20- volt,  60- 
cycle,  4-pole,  lo-horsepower  single-phase  induction  motor.  The 
fundamental  data  employed  in  the  construction  of  this  diagram 


230      ELECTRIC  MOTORS,    THEIR   ACTION   AND    CONTROL. 


were  derived  by  test,  and  are  as  follows:  stator  resistance,  .304 
ohm;  current  with  motor  running  free,  19  amperes;  corresponding 
input,  i  k.w.;  and  power  factor  24  per  cent.  The  current  with 
rotor  at  standstill  is  170  amperes;  input,  13.4  k.w.;  power  factor, 
36  per  cent.  Line  potential  in  both  instances  is  220  volts. 

The  values  derived  from  the  diagram  are  given  in  the  following 
table  and  presented  in  the  form  of  curves  in  Fig.  134. 

CHARACTERISTICS  OF   A  220-VOLT,    60-CYCLE,    10-HORSE- 
POWER,   SINGLE-PHASE   INDUCTION  MOTOR. 


Point. 

Amp. 

%  P.F. 

K.W.  Input. 

H.P.  Output. 

%  Eff. 

R.P.M. 

Ft.-lbs. 
Torque. 

M 

19 

24 

1 

0 

0 

1800 

0 

1 

25 

65 

3.56 

3.4 

70 

1782 

10 

2 

30 

75 

4.95 

5.03 

76 

1772 

15 

3 

40 

81 

7.15 

7.65 

80 

1760 

23 

P 

50 

83 

9.24 

10.0 

81 

1745 

30 

5 

60 

85 

11.2 

12.0 

80 

1738 

36 

6 

80 

81 

14.2 

14.75 

78 

1715 

45 

7 

100 

78 

17.2 

16.5 

72 

1690 

51 

8 

119 

70 

18.7 

16.0 

64 

1640 

51 

9 

140 

61 

18.8 

13.1 

52 

1550 

44.5 

10 

155 

51 

17.6 

8.8 

37.5 

1400 

33.0 

F 

170 

36 

13.4 

0 

0 

0 

0 

Comparison  of  these  characteristic  curves  of  the  single-phase 
induction  motor  with  those  of  the  standard  polyphase  induction 
motor  brings  out  the  fact  that  the  former  has  zero  torque  not  only 
at  synchronous  speed  but  also  at  standstill,  whereas  the  latter  has 
a  starting  torque  in  excess  of  that  developed  at  rated  load. 

The  following  table  gives  characteristics  of  operation  attained  by 
standard  single-phase  induction  motors. 

DATA  OF   STANDARD   SINGLE-PHASE  INDUCTION  MOTORS. 
110  TO  440  VOLTS. 


Per  cent  Power 

Per  cent. 

H.P. 

Poles. 

Per  cent 
Slip. 

Pull-out 
Torque.* 

Factor  Load. 

Efficiency  Load. 

* 

f 

Rtd. 

l\ 

% 

1 

Rtd. 

H 

\ 

4 

6 

.5 

46 

58 

66 

68 

53 

60 

63 

60 

4 

4 

.6 

55 

59 

73 

75 

60 

63 

68 

62 

2 

4 

2.5 

.8 

56 

65 

77 

76 

71 

75 

78 

77 

5 

4 

2.5 

.8 

78 

83 

86 

86 

71 

76 

77 

76 

10 

4 

2.5 

.8 

75 

81 

84 

83 

75 

79 

80 

79 

20 

6 

2 

.9 

78 

80 

86 

87 

85 

88 

86 

85 

30 

8 

2 

.9 

68 

80 

85 

84 

77 

81 

83 

82 

50 

4 

2.3 

2.0 

91 

94 

93 

91 

82 

84 

86 

86 

*  Pull-out  torque  in  terms  of  rated  load  torque. 


THE  SINGLE-PHASE  INDUCTION   MOTOR.  231 

Comparison  of  the  values  of  this  table  with  the  corresponding  one 
on  p.  193  shows  that  in  general  the  power  factor,  efficiency  and 
pull-out  torque  are  higher  for  polyphase  than  for  single-phase 
motors,  while  the  speed  regulation  of  the  single-phase  machine  is 
better.  This  latter  feature  of  the  single-phase  induction  motor 
is  accounted  for  by  Dr.  C.  P.  Steinmetz  as  follows :  * 

"  Since  in  the  single-phase  motor  one  primary  circuit  and  a  multiplicity  of  second- 
ary circuits  exist,  all  secondary  circuits  are  to  be  considered  as  corresponding  to  the 
same  primary.  Thus  the  joint  impedance  of  all  secondary  circuits  must  be  used  as 
the  secondary  impedance,  at  least  at  or  near  synchronism.  Thus,  if  the  armature  has  a 
quarter-phase  winding  of  impedance  Zl  per  circuit,  the  resultant  secondary  impedance 

is  — ;  if  it  contains  a  three-phase  winding  of  impedance  Zj  per  circuit,  the  resulting 

9 

secondary  impedance  is  — .  In  consequence  thereof,  the  resulting  secondary  im- 
pedance of  a  single-phase  motor  is  less  in  comparison  with  the  primary  impedance 
than  in  the  polyphase  motor.  Since  the  drop  in  speed  under  load  depends  upon  the 
secondary  resistance,  that  occurring  in  the  single-phase  induction  motor  is  generally  less 
than  with  the  polyphase  motor." 

Methods  of  Starting.  —  As  already  shown  (pp.  215  et  seq.),  the 
simple  single-phase  induction  motor  cannot  exert  any  starting  torque. 
In  practice,  however,  except  in  the  smallest  sizes  which  may  be 
started  by  hand,  the  conditions  of  service  which  this  motor  is  to 
meet  require  a  starting  torque  as  high  as  150  per  cent  of  the  rated 
value,  consequently  some  device  producing  this  feature  must  be 
connected  with  or  incorporated  into  the  machine.  The  methods 
of  accomplishing  this  result  may  be  grouped  into  two  general 
classes.  The  first  is  technically  known  as  phase-splitting  and  the 
second  as  the  repulsion-motor  method. 

Split-phase  Starting.  —  Two-phase  currents  may  be  obtained  on 
a  single-phase  circuit  by  dividing  it  into  two  branches  one  of  which 
is  inductive  and  the  other  non-inductive.  If  supplied  with  two- 
phase  currents,  even  though  these  be  less  than  90  degrees  apart,  an 
induction  motor  is  self -starting;  thus  when  synchronous  speed  is 
approximated  the  phase-splitting  device  may  be  cut  out  and  the 
machine  will  continue  to  operate.  There  are  many  ways  to  obtain 
such  split  currents.  The  two  parts  of  the  circuit  may  be  in  series, 
one  being  shunted  by  inductance  or  capacity  (Fig.  135).  They  may 
also  be  put  into  inductive  relation  to  each  other  to  produce  a  phase 
difference,  f 

*  Elements  of  Electrical  Engineering  1902,  p.  284. 

f  U.  S.  Patent  No.  401,520,  April  16,  1889,  to  Nickola  Tesla. 


232      ELECTRIC   MOTORS,    THEIR   ACTION   AND    CONTROL. 


Motors  employing  the  above  starting  methods  are  provided  with 
two  stator  windings,  a  working  winding  and  a  starting  winding. 
The  two  windings  are  displaced  from  each  other  by  about  ninety 
magnetic  degrees,  just  as  in  the  ordinary  two-phase  motor.  The 


FIG.    135. — SLIP  PHASE  CIRCUIT,  USING  INDUCTANCE  AND  RESISTANCE. 

working  winding,  however,  is  of  more  turns,  being  spread  over  a 
larger  surface,  and  of  heavier  wire  than  the  starting  winding,  because 
it  remains  in  circuit  as  long  as  the  motor  operates,  whereas  the 
starting  coils  are  only  in  use  momentarily. 


^ 

-°~I 

3 

FIG.      136.   — CONNECTIONS      FOR      START-     FIG.  137. —PHASE-SPLITTING  METHOD 
ING     SMALL     SINGLE-PHASE    INDUCTION  DEVISED   BY    BROWN,  BOVERI    FOR 

MOTORS.  USE  WITH  LARGE  MOTORS. 

The  method  illustrated  in  Fig.  136  has  been  developed  by 
Brown,  Boveri  and  Co.  of  Baden,  Switzerland.  At  starting  the 
two  windings  are  placed  in  series  across  the  supply  lines,  the 
starting  winding  5  being  shunted  by  the  condenser.  The  current 


THE  SINGLE-PHASE  INDUCTION   MOTOR. 


233 


consequently  lags  more  in  that  winding,  the  difference  in  phase 
between  the  currents  in  R  and  S  being  sufficient  to  set  up  an  ellip- 
tically  formed  rotating  field.  The  starting  winding  and  its  con- 
denser are  cut  out,  and  the  working  winding  placed  directly  across 


D 


FIG.  138. —  CONNECTIONS  OF  GENERAL  ELECTRIC  COMPANY  CONDENSER  COM- 
PENSATOR FOR  PHASE-SPLITTING  AND  STARTING  SINGLE -PHASE  INDUCTION 
MOTORS. 

the  line  by  means  of  the  double-throw  switch  T  when  the  motor 
has  approximately  attained  synchronous  speed.  This  method  is 
slightly  modified  when  machines  of  over  5-h.p.  capacity  are  to 
be  started.  The  two  windings  in  such  instances  are  placed  in 
parallel,  as  shown  in  Fig.  137.  By  this  means  the  working  coil 
circuit  is  not  broken  and  the  flash  occurring  upon  cutting  out  the 
auxiliary  winding  is  eliminated. 

An  excellent  method  for  starting  single-phase  motors  has  been 
developed  by  the  General  Electric  Company  under  patents  granted 


234      ELECTRIC  MOTORS,    THEIR  ACTION  AND    CONTROL. 

to  Dr.  C.  P.  Steinmetz,  the  connections  for  which  are  substan- 
tially as  shown  in  Fig.  138.*  Two  terminals  of  the  stator  winding, 
which  is  substantially  of  standard  three-phase  construction,  are 
connected  directly  to  the  supply  lines.  The  third  terminal  is  also 
connected  to  either  one  of  the  mains  through  an  auto-transformer 
(p.  194),  the  order  depending  upon  the  direction  of  rotation 
desired.  The  ends  of  this  compensator  are  placed  across  a  con- 
denser. This  combination  is  technically  known  as  a  condenser-com- 
pensator, and  is  employed  because  a  condenser  of  given  volt-ampere 


FIG.   139. — ARRANGEMENT  OF  WORKING  AND  AUXILIARY  STATOR  COILS,  HEY- 
LAND  SELF-STARTING  SINGLE-PHASE  INDUCTION  MOTOR. 

capacity  is  more  economically  constructed  for  high  than  for  low 
voltage.  The  starting  winding  can  be  cut  out  by  opening  the  switch 
at  S  after  the  motor  is  up  to  speed.  It  may,  however,  be  advan- 
tageous to  keep  the  starting  coil  in  circuit,  if  of  sufficient  current 
capacity  for  continuous  service,  because  the  increased  power  factor 
at  light  loads  thus  obtained  more  than  compensates  for  the  losses 
occurring  in  the  transformer. 

The  use  of  external  phase-splitting  apparatus  may,  however,  be 
dispensed  with  if  the  two  stator  windings  are  arranged  to  have 
different  time  constants.  This  is  accomplished  by  having  the 
auxiliary  winding  of  larger  self -inductance  than  the  main  coil. 

*  U.  S.  Patent  Nos.  602,920  and  602,921,  April  26,  1898. 


THE  SINGLE-PHASE  INDUCTION   MOTOR.  235 

Heyland  devised  a  very  successful  motor  of  this  type,  utilizing  the 
scheme  suggested  in  the  Tesla  patent  cited  above  (page  231). 
The  working  winding  P  is  distributed  in  a  series  of  semi- 
closed  slots.  The  starting  coils  S  are  short-circuited  upon 
themselves  and  placed  in  closed  ducts,  the  result  being  a  highly 
inductive  secondary  circuit,  the  general  arrangement  being  as  illus- 
trated in  Fig.  139.  The  current  induced  in  the  secondary  winding 
lags  almost  90  degrees  with  respect  to  the  primary  current,  pro- 
ducing a  field  component  similar  to  that  caused  by  the  second 
phase  of  a  two-phase  current.  The  starting  torque  thus  produced 
is  large,  though  the  power  factor  of  the  machine  is  necessarily  low, 
and  therefore  the  starting  coil  should  be  cut  out  as  soon  as  the 
machine  has  come  up  to  speed.* 

The  rotor  windings  employed  in  connection  with  any  or  all  of  the 
preceding  methods  for  starting  may  be  of  the  standard  squirrel- 
cage  or  slip-ring  type. 

Repulsion  Motor  Starting.  —  A  very  interesting  type  of  self- 
starting  single-phase  induction  motor  is  manufactured  by  the 
Wagner  Electric  Mfg.  Co.  of  St.  Louis,  f  This  motor  is  provided 
with  an  armature  of  the  ordinary  direct-current  drum  type, 
having  a  disk  commutator  with  radial  bars.  The  brushes  bear- 
ing upon  the  commutator  are  displaced  about  45  degrees  from 
the  corresponding  neutral  zones  and  short-circuited  upon  each 
other.  The  stator  winding  is  connected  to  the  supply  lines,  and 
at  starting  the  machine  speeds  up  as  a  repulsion  motor  (p.  257). 
In  the  annular  space  between  the  armature  core  and  the  shaft  are 
two  governor  weights  w  (Fig.  140),  which  are  forced  outward,  further 
and  further,  by  centrifugal  force  as  the  machine  accelerates.  When 
synchronous  speed  is  nearly  attained  the  force  acting  upon  these 
weights  is  sufficient  to  push  the  heavy  copper  ring  R,  against  the 
action  of  spring  S,  into  contact  with  the  inner  cylindrical  surface 
of  the  commutator  bars  G,  thus  completely  short-circuiting  the 
armature  winding.  Simultaneously  with  this  action  the  sleeve  P 
is  forced  to  the  left  sufficiently  to  lift  the  brushes  B  from  the  com- 
mutator. This  series  of  automatic  actions  transforms  the  machine 
from  a  repulsion  to  a  single-phase  induction  motor,  having  in  the 
latter  form  what  is  substantially  a  squirrel-cage  armature  winding. 

*  Electrical  Engineer,  Vol.  XXXVI,  p.  306.     London,  1896. 
t  U.  S.  Patent  No.  543,836,  December  4,  1894. 


236      ELECTRIC  MOTORS,   THEIR  ACTION  AND   CONTROL. 


FIG.   140. —GENERAL  ARRANGEMENT  OF  WAGNER  MOTOR  SHOWING  AUTOMATIC 
SHUNT-CIRCUITING  DEVICES. 


The  starting  torque  thus  obtained  may  be  readily  adjusted  to 
about  twice  the  normal  value,  without  an  excessive  current  being 
required. 

A  very  interesting  feature  of  the  Wagner  motor,  and  one  equally 
pertinent  to  repulsion  motors,  is  the  relation  between  torque  and 
thickness  of  rotor  brushes.  The  series  of  curves  shown  in 
Fig.  141  were  determined  from  tests  of  a  5-h.p.,  220-volt  Wagner 
motor.  Curve  A  shows  the  speed-torque  relation  on  accelerating 
with  normal  brush  thickness,  this  being  substantially  that  of  a 
commutator  bar.  Curve  B  represents  the  relations  existing  with 
a  brush  of  twice  normal  thickness,  etc.  It  is  apparent  from  these 
curves  that  the  normal  thickness  of  brush  gives  the  highest  starting 
and  synchronous  speed  torques.  Further  study  of  Fig.  141  indicates 
that  use  of  a  brush  thinner  than  normal  might  tend  to  produce  start- 
ing and  synchronous  speed  torques  of  greater  value  than  occur  with 
normal  brush  thickness.  Practical  questions,  however,  as  regards 
mechanical  strength  limit  the  reduction  of  brush  thickness. 

Single-phase  induction  motors,  in  addition  to  being  provided 
with  one  or  another  of  the  preceding  means  for  developing 
starting  torque,  require,  when  over  moderate  size  (3  or  5  h.p.) 


THE  SINGLE-PHASE  INDUCTION  MOTOR. 


23T 


70 


60 


50 


40 


if  30 


Normal  Thidkness 
Doubl 


D 


Triple 
Quadruple 


B 


20 


10 


Rat  ed  Lo$d  Torque 


0   200   400   600   800  1000  1200  1400  1600  1800 

R.P.M. 

FIG.   141.  —  SPEED-TORQUE  CURVES  WITH  VARIOUS  BRUSH  THICKNESSES. 


the  introduction  of  starting  compensators  (p.  195),  or  use  of  wound 
rotors  with  slip-ring  control  (p.  199).  This  precaution  is  neces- 
sary, as  the  inrush  current  otherwise  occurring  would  be  consider- 
able and  likely  to  react  upon  the  line,  producing  voltage  fluctuations. 

For  further  information  on  single-phase  induction  motors,  the  reader  is  referred  to 
the  following: 

ALTERNATING-CURRENT  MOTORS.    A.  S.  McAllister.     New  York,  1909. 

SINGLE-PHASE  INDUCTION  MOTOR.  A.  Still.  Elect.  World,  New  York,  1906. 
Vol.  XLIII,  pp.  1108,  1152,  1182,  1202. 

SINGLE-PHASE  INDUCTION  MOTORS.  Dr.  C.  P.  Steinmetz.  Trans.  A.  I.E.  E., 
Vol.  XV,  1898,  pp.  35-110. 

SINGLE-PHASE  INDUCTION  MOTORS.  W.  S.  Franklin.  Trans.  A.I.  E.E.,  Vol. 
XXIII,  1904,  p.  429. 

THEORY  OF  SINGLE-PHASE  INDUCTION  MOTOR.  V.  A.  Finn.  Elect.  Rev.,  London, 
February,  1906. 

WECHSELSTROMTECHNIK.  Vol.  V,  by  Arnold  and  La  Cour,  pp.  112  and  275.  Ber- 
lin, 1909, 


CHAPTER    XVIII. 

COMMUTATING  ALTERNATING-CURRENT  MOTORS. 

COMMUTATING  alternating-current  motors  are  those  having  a 
closed-coil  armature  provided  with  a  commutator,  being  similar  to 
d.c.  motors  in  general  construction.  Fundamentally  there  are  three 
such  types,  namely,  shunt,  series,  and  repulsion  motors,  but  many 
modifications  and  combinations  have  been  devised,  the  more 
important  of  which  will  be  considered  later.  The  general  histor- 
ical facts  concerning  these  and  other  a.c.  motors  have  already  been 
given  in  Chapter  XII. 

Alternating  Current  Shunt  Motor.  —  Owing  to  the  fact  that  the 
direction  of  rotation  of  any  d.c.  motor  is  the  same  irrespective  of  the 
direction  of  current  supply,  early  attempts  were  made  to  adapt  such 
machines  for  service  on  alternating-current  systems.  Of  course 
these  motors  must  be  provided  with  laminated  field  frames  to  reduce 
the  great  losses  due  to  eddy  currents  that  would  otherwise  occur. 
The  simple  shunt  motor  of  this  type  is  not,  however,  of  any  com- 
mercial value,  for  the  following  reasons: 

1.  Low  power  factor,  due  to  the  many  turns  of  the  field  winding, 
the  inductance  of  which  is  extremely  large. 

2.  Severe  sparking  at  the  brushes,  due  to  the  fact  that  as  each 
coil  passes  under  a  brush  it  becomes  a  short-circuited  secondary 
of  a  transformer,  and  thus  a  seat  of  heavy  currents. 

3.  Low  weight  efficiency.     This  trouble  is  caused  by  the  phase 
difference  between  armature  and  field  currents  and  corresponding 
relation  between  their  respective  fluxes,  resulting  in  greatly  reduced 
torque. 

The  inductance  of  the  armature  is  small  with  respect  to  that  of 
the  shunt-field  circuit,  hence,  the  two  currents  differ  considerably 
in  phase.  Let  the  vector  OB  (Fig.  142)  represent  the  current  in  the 
armature  circuit  and  d1  its  small  angle  of  lag  with  respect  to  the  line 
e.m.f.  OAj  while  AC  represents  the  field  current  and  62  its  large 
angle  of  lag;  then  j>  =  62  -  6l  is  the  angular  difference  between  OC 

238 


COMMUTATING  ALTERNATING-CURRENT  MOTORS.         239 

and  AB,  the  field  and  armature  currents  (also  fluxes),  respectively. 
The  relation  between  these  currents  is  also  shown  in  the  wave 
diagram  of  Fig.  142.  The  torque  at  any  moment  is  t  =  KIa  sin 
OJf  sin  02,  which  by  reduction  gives  T  =  Klalf  cos  <£  as  the  effective 
value.  In  other  words,  the  torque  developed  by  an  a.c.  shunt  motor  is 

A 
,C  / ^ 

'  C 


FIG.   142.  —  VECTOR  AND  WAVE  DIAGRAMS  OF  E.M.F.   AND  CURRENT  OF 
A.C.  SHUNT  MOTOR. 

dependent  not  only  upon  the  field  and  armature  currents,  but  also 
upon  the  cosine  of  the  angle  between  them. 

If  the  currents  were  in  phase  cos  <j>  would  be  unity  and  the 
torque  of  the  a.c.  shunt  motor  would  be  equivalent  to  that  of  the 
d.c.  machine.  Economy  of  design  and  efficiency  of  operation 
demand  a  shunt-field  winding  of  many  turns  and  an  armature  wind- 
ing of  comparatively  few  turns,  so  that  <j>,  which  is  the  phase  differ- 
ence between  field  and  armature  currents,  will  always  be  large. 
Hence,  as  already  stated,  the  torque  and  power  per  unit  of  weight 
of  such  an  a.c.  shunt  motor  are  necessarily  small. 

The  a.c.  shunt  motor  may  be  used  with  greatly  increased  weight 
efficiency  on  two-phase  circuits,  by  supplying  the  field  winding 
from  the  leading  phase,  and  the  armature  from  the  lagging  phase. 
The  lag  of  field  current  would  bring  it  very  closely  in  phase  with 
the  armature  current;  thus  <j>  would  be  small  and  the  torque  corre- 
spondingly increased  with  the  same  values  of  Ia  and  //.  Another 
advantage  of  this  scheme  is  that  adjustable  voltage  speed  control 
becomes  available  through  the  simple  introducing  of  a  variable 
ratio  auto-transformer  in  armature  circuit.  This  modification  of 
the  shunt  motor,  however,  has  not  been  adopted  to  any  extent  in 
practice,  because  induction  motors  on  account  of  their  higher  power 
factor  and  better  balanced  condition  would  naturally  be  used  when 
two-phase  currents  are  available. 


240      ELECTRIC  MOTORS,    THEIR  ACTION   AND    CONTROL. 

The  Alternating-current  Series  Motor.  —  The  powerful  starting 
torque  and  adaptation  of  speed  to  load  characteristics  of  the  d.c. 
series  motor  make  this  type  particularly  suitable  for  traction  and 
many  other  uses  requiring  such  qualities.  The  limitations  of  direct 
current  generation  and  transmission,  as  well  as  special  advantages 
of  a.c.  voltage  control,  have  made  the  development  of  a  correspond- 
ing a.c.  motor  very  desirable,  a  fact  appreciated  for  many  years.* 

Synchronous  motors,  whether  single  or  polyphase,  and  single- 
phase  induction  motors,  all  having  little  or  no  starting  torque,  are 
obviously  unsuited  to  railway  and  many  other  purposes.  Poly- 
phase induction  motors  require  at  least  three  supply  conductors  and 
are  for  that  reason  less  desirable  than  single-phase  apparatus, 
especially  for  traction.  Hence  a  single-phase  motor  with  a  power- 
ful starting  torque  has  an  enormous  field  of  usefulness.  Up  to  the 
present  time  the  series  and  repulsion  motors  both  with  commu- 
tators are  the  only  a.c.  types  fulfilling  this  condition. 

The  operation  of  series  motors  on  the  high  frequency  circuits 
formerly  employed  (100  to  133  p.p.s.)  was  attempted  at  various 
times,  but  not  with  success,  except  in  the  case  of  very  small  machines. 
This  failure  was  due  to  excessive  transformer  action  in  the  coils  of 
the  armature  winding,  short-circuited  during  commutation,  as  well 
as  to  the  low  power  factor  caused  by  the  large  reactance  of  the  field 
windings. 

The  introduction  and  use  of  low  frequency  (25  p.p.s.)  systems 
for  power  transmission  is  the  basis  for  the  later  commercial  develop- 
ment of  the  a.c.  series  motor.  In  1902,  Mr.  G.  B.  Lamme,  of  the 
Westinghouse  Elect.  &  Mfg.  Co.,  called  attention  to  an  a.c.  series 
motor  which  operated  on  circuits  having  a  frequency  of  i6J  p.p.s.t 
This  motor  had  a  powerful  starting  torque,  high  power  factor,  and 
was  of  relatively  high  efficiency,  but  the  low  frequency  necessary 
for  its  proper  operation  unsuited  it  for  service  on  circuits  of  stand- 
ard frequency.  Furthermore,  illumination  by  arc  or  incandescent 
lamps  at  i6§  cycles  is  not  satisfactory.  The  design  has  since  then 
been  modified  to  adapt  this  type  of  motor  to  the  standard  frequency 
of  25  p.p.s. 

The  same  current  flows  through  both  field  and  armature  windings 
of  an  a.c.  as  well  as  d.c.  series  motor,  hence  there  can  be  no  phase 

*  Alex.  Siemans,  Journal  British  Inst.  of  Elect.  Engs.,  p.  527,  Vol.  XIII,  1884. 
f  Transactions  A.  I.  E.  E.,  pp.  10-49,  Vol.  XX,  1902. 


COMMUTATING  ALTERNATING-CURRENT   MOTORS.         241 

difference  between  field  and  armature  currents.  In  this  respect  it 
differs  from  the  a.c.  shunt  motor  whose  torque  depends  not  only  upon 
field  and  armature  currents  but  also  upon  the  cosine  of  the  phase 
angle  between  them,  as  already  explained.  There  are,  however, 
other  limitations  of  the  a.c.  series  motor  and  special  features  of 
design  are  found  necessary  by  reason  of  the  following  phenomena, 
peculiar  to  a.c.  machines: . 

1.  Iron  losses  throughout  the  magnetic  circuit,  due  to  alterna- 
tions of  the  flux. 

2.  An  e.m.f.  generated  in  the  armature  windings  by  the  alter- 
nating magnetic  field,  and  defined  as  a  transformer  e.m.f.  in  con- 
tradistinction to  the  voltage  developed  by  armature  rotation. 

3.  A  local  current  circulating  in  those  coils  short-circuited  by 
the  brushes.      This  current  is  due  to  the  transformer  e.m.f.  of 
No.  2. 

4.  An  e.m.f.  of  self-induction  in  the  field  and  armature  windings. 

5.  Power  factor  less  than  unity,  due  to  inductance  of  the  wind- 
ings. 

i/  Iron  Losses.  The  total  iron  losses  occurring  in  the  a.c.  series 
motor  may  be  divided  into  two  parts :  that  taking  place  in  the  arma- 
ture coil  and  polar  faces  due  to  the  rotation  of  the  former,  and  that 
occurring  in  the  entire  magnetic  circuit,  due  to  the  alternations  of 
the  magnetic  flux.  The  losses  arising  from  rotation  of  the  armature 
coil,  being  common  to  both  a.c.  and  d.c.  motors,  are  often  called 
"d.c.  iron  losses."  These  are  supplied  mechanically  and  act  like 
the  resisting  torque  of  friction.  The  losses  caused  by  flux  alter- 
nation are  supplied  electrically  and  are  due  to  eddy  currents  and 
hysteresis,  the  former  being  reduced  to  a  reasonable  amount  by 
employing  laminated  field  magnets  as  well  as  a  laminated  armature 
coil.  A  reduction  in  hysteresis  loss  is  possible  by  operating  at  low 
flux  densities.  Hence  in  the  case  of  a  series  motor  designed  for 
a.c.  service  a  wholly  laminated  magnetic  circuit  is  necessary.  This 
feature,  combined  with  the  limitation  of  flux  densities,  results  in  a 
total  weight  30  to  50  per  cent  greater  than  that  of  a  corresponding  d.c. 
machine.  The  overall  dimensions  are  also  proportionately  larger. 

2.  Transformer  e.m.f.  In  addition  to  the  c.e.m.f.  of  rotation  a 
second  e.m.f.  is  generated  in  the  armature  winding,  which  does  not, 
however,  appear  at  the  brushes,  except  locally  as  a  cause  of  sparking, 
explained  in  3.  The  rate  of  cutting  lines  of  force  due  to  armature 


242      ELECTRIC  MOTORS,    THEIR   ACTION   AND    CONTROL. 

rotation  is  a  maximum  at  the  position  of  coils  A  and  B,  Fig.  143, 
while  the  minimum  rate  of  cutting  occurs  while  the  coils  are  in  the 
so-called  "neutral"  position,  CD.  The  e.m.f.  generated  by  the 
armature  rotation  tends  to  cause  current  flow  from  coil  D  upwards 
through  each  half  of  the  armature  winding  to  the  coil  C,  producing 
poles  at  C  and  D.  The  second  or  transformer  e.m.f.  is  produced  by 
the  alternations  of  flux  passing  through  the  armature  coil.  For 
example,  in  the  case  of  a  ring  wound  armature  placed  in  a  bipolar 


FIG.   143. — ARMATURE  OF  SERIES  MOTOR  SHOWING  DIRECTION  OF  C.E.M.F. 

field,  one-half  of  the  total  flux  passes  through  the  sections  at  C  and 
D  respectively,  hence  maximum  flux  variation  occurs  there.  On 
the  contrary,  no  lines  of  force  pass  through  coils  A  and  B,  so  that  no 
transformer  action  occurs  and  no  e.m.f.  is  induced  in  them.  This 
transformer  e.m.f.  produces  no  difference  of  potential  at  the  brushes 
placed  vertically  in  their  usual  position  as  indicated,  because  the 
portions  of  the  armature  winding  in  connection  with  them  are  of 
equal  potential,  the  transformer  e.m.f.  being  shown  by  heavy 
arrows  (Fig.  144).  It  is  evident  that  this  e.m.f.  does  not  act  against 
the  flow  of  current  from  the  supply  lines  and  produces  no  effect, 
except  that  the  particular  coils  short-circuited  by  the  brushes 


COMMUTATING   ALTERNATING-CURRENT   MOTORS. 


243 


experience  maximum  transformer  induction,   which   tends   to   set 
up  large  currents  in  them,  as  already  explained  under  heading  3. 
The  value  of  this  transformer  e.m.f.  in  the  armature  is 


4- 


10 


(64) 


this  being  the  well-known  expression  for  transformer  e.m.f.  in  which 
/  is  the  frequency,  &m  the  maximum  armature  flux  due  to  the  field 
m.m.f.,  and  S  the  equivalent  number  of  armature  turns.  If  the 
total  number  of  armature  conductors,  counting  all  around  the  per- 


FIG.   144. — DIRECTION  OF  TRANSFORMER  E.M.F.  INDUCED  IN  ARMATURE 
WINDING  BY  AN  A.C.  FIELD. 


iphery,  is  Sa,  the  turns  in  series  are  Sa  -*-  2  on  the  bipolar  ring  arma- 
ture in  Fig.  144,  which  is  simpler  than  a  drum  winding  to  represent 
and  study.  As  already  stated,  the  field  flux  linked  with  each  turn 
of  armature  winding  depends  upon  its  position,  being  proportional 
to  the  cosine  of  its  angular  displacement  from  the  line  CD.  The 

fy 

average  value  of  the  cosine  in  each  quadrant  is  -  ,  and  each  turn  in  a 

ring  armature  carries  only  one-half  of  the  flux,  hence  the  effect  of 
Sa  total  conductors  is  equivalent  to 

_  Sa  m     2  Sa  ^ 

2       27T          271 


244       ELECTRIC   MOTORS,    THEIR   ACTION   AND    CONTROL. 

Substituting  this  value  of  S  in  equa.  64,  the  transformer  e.m.f. 
induced  in  the  armature  winding  by  the  alternating  field  flux,  and 
lagging  one-quarter  period  or  90  degrees  with  respect  to  it,  is 


This  same  formula  applies  equally  well  to  a  drum  armature,  in  which 
case  the  turns  are  only  one-half  as  many  as  the  total  conductors, 
but  the  flux  is  not  divided  between  two  turns  as  in  a  ring  winding. 
Hence  these  two  factors  would  cancel  out  if  introduced  in  equation. 
3.  Local  Armature  Current.  Since  those  coils  undergoing  com- 
mutation are  the  seat  of  maximum  transformer  action,  a  large  cur- 
rent will  flow  in  them  as  in  any  short-circuited  secondary.  The 
flux  due  to  this  secondary  current  being  in  opposition  to  the  primary 
flux  tends  to  weaken  the  field  just  when  and  where  its  greatest 
strength  is  required  for  commutation.  This  local  current  may  be 
greatly  in  excess  (5  to  15  times)  of  the  normal  armature  current, 
thus  producing  local  heating  as  well  as  an  additional  current  and 
PR  loss  in  the  primary  (i.e.,  field)  winding.*  The  sudden  inter- 
ruption of  the  heavy  current  in  the  short-circuited  coil  also  draws  a 
large  spark  at  the  brushes  and  causes  commutator  troubles.  Since 
the  transformer  e.m.f.  and  current  depend  upon  the  frequency  of 
flux  alternations,  the  number  of  turns  in,  and  resistance  of,  each  short- 
circuited  coil,  they  can  be  reduced  by  proper  modification  of  these 
three  factors.  That  is,  frequency  of  the  supply  circuit  should  be 
as  low  as  possible,  consistent  with  standard  practice,  and  the  num- 
ber of  turns  of  wire  or  inductors  in  series  between  consecutive 
commutator  bars  should  be  small,  ihe  transformer  e.m.f.  being 
directly  proportional  to  these  two  factors.  This  latter  condition 
increases  the  number  of  armature  sections  as  well  as  commutator 
bars,  and  therefore  cost  of  construction,  but  greatly  diminishes  the 
tendency  to  spark,  self-induction  being  proportional  to  the  square 
of  the  number  of  turns  of  wire.  The  e.m.f.  is  reduced,  as  just 
shown,  while  the  resistance  of  the  coil  circuit  is  increased  by  employ- 
ing brushes  of  higher  contact  resistance  than  is  usual  with  d.c. 
machines,  or  by  inserting  high  resistance  connectors  or  preventive 
leads  (P)  of  German  silver  between  the  armature  sections  and  the 
commutator  bars  (Fig.  145).  The  high  resistance  of  the  special 

*  Electric  Journal,  p.  7,  Vol.  VI,  1909. 


COMMUTATING   ALTERNATING-CURRENT   MOTORS.          245 

brush  contacts  or  of  the  preventive  leads  is  only  a  small  factor  of  the 
resistance  of  the  whole  armature  winding,  consequently  it  does  not 
lower  the  efficiency  of  the  machine  to  any  marked  extent.  The  ad- 
ditional resistance  of  the  preventive  leads  is,  however,  a  considerable 


FIG.   145. — PREVENTIVE  LEADS. 

factor  of  the  local  circuit,  hence  it  is  very  effective  in  cutting  down 
the  value  of  the  local  or  short-circuit  current. 

4.  E.M.F.  of  Self-induction  occurring  in  both  field  and  armature 
windings.  This  e.m.f.  is  due  to  the  alternating  flux,  and  introduces 
a  factor  not  existing  in  the  d.c.  motor,  that  is,  in  addition  to  the 
c.e.m.f.  of  rotation,  the  impressed  voltage  must  overcome  an  e.m.f. 
of  self-induction  equal  to  a>LI.  To  cause  an  equal  current  to 
flow,  with  other  conditions  the  same,  the  applied  e.m.f.  must  there- 
fore be  greater  than  in  the  d.c.  machine.  As  a  matter  of  fact,  the 
actual  voltage  supplied  to  the  terminals  of  an  a.c.  series  railway 
motor  is  usually  lower  than  for  the  d.c.,  being  about  250  compared 
with  about  550  volts,  hence  the  e.m.f.  of  rotation  must  be  relatively 
still  lower  in  the  former  machine.  In  other  words,  the  a.c.  machine 
is  designed  for  a  lower  voltage.  Another  difficulty  due  to  this 
inductive  e.m.f.  arises  when  one  or  more  field  turns  become  short- 
circuited,  forming  a  closed  secondary  circuit  and  drawing  excessive 
current,  very  likely  to  result  in  a  burned  out  field  winding.  The 
transformer  e.m.f.  set  up  in  the  armature  of  a  bipolar  machine, 
as  given  in  equa.  65,  is  equal  to  the  full  c.e.m.f.  developed  by  rotation 
when  the  line  frequency  equals  motor  r.p.m.  -*-  60,  which  relation 
corresponds  to  synchronous  speed.  This  is  evident  because  the 
armature  turns  cut  the  flux  at  the  same  rate  in  both  cases.  Hence 
the  effect  of  a  short  circuit  in  the  armature  is  aggravated  in  the  series 
a.c.  motor. 


246      ELECTRIC  MOTORS,    THEIR   ACTION   AND    CONTROL. 

5.  Power  Factor.  Since  the  field  winding  is  highly  inductive, 
and  the  armature  winding  may  have  considerable  inductance,  the 
current  flowing  through  them  is  not  in  phase  with  the  line  e.m.f. 
This  condition  does  not  affect  the  torque  of  the  motor,  but  the  result- 
ing low  power  factor  impairs  the  regulation  of  the  alternators,  trans- 
formers and  line,  at  the  same  time  lowering  the  efficiency  of  the 
entire  system.  This  e.m.f.  of  self-induction  in  the  field  winding  is 
directly  proportional  to  the  frequency  of  the  supply  voltage,  to  the 
field  flux  and  to  the  square  of  the  number  of  its  turns,  hence  in  any 
attempt  to  improve  the  power  factor  of  the  motor,  these  various 
factors  must  all  be  considered. 

a.  The  frequency  cannot  be  lowered  indefinitely,  since  the  motor 
must  operate  on  existing  circuits,  few  of  which  have  a  frequency  of 
less  than  25  p.p.s.     Many  engineers  are  agitating  the  use  of  i5-cycle 
circuits  for  series  motors,  hoping  in  this  way  to  improve  the  power 
factor,  and  while  that  result  undoubtedly  would  be  thus  obtained, 
the  greater  cost  of  the  alternators  and  transformers  might  readily 
offset  this  gain. 

b.  Total  Flux.    By  increasing  the  number  of  inductors  and  at 
the  same  time  the  number  of  sections  on  the  armature,  the  field 
flux  can  be  reduced  and  the  torque  maintained  constant.     That 
is,  the  armature  is  made  strong  with  respect  to  the  field,  so  that  the 
field  inductance  can  be  decreased.     Hence  in  practice  the  total  flux 
of  the  a.c.  series  motor  at  corresponding  current  values  is  not  as 
high  as  in  the  d.c.  machine.      The  increase  in  armature  inductance 
is  eliminated  by  "  compensation,"  as  explained  later. 

c.  Turns  in  the  Field  Coil.     These  can  be  kept  down,  first,  by 
having  steel  of  high  permeability,  and  secondly  by  having  a  some- 
what shorter  air  gap,  thus  obtaining  the  necessary  flux  with  a  smaller 
number  of  ampere-turns.     In  fact,  the  lower  value  of  flux  indicated 
above  (b)  tends  to  reduce  the  number  of  field  turns  required. 

It  has  been  suggested  that  the  power  factor  of  the  series  motor 
could  be  improved  by  increasing  its  resistance  or  decreasing  the 
ratio  coL  -5-  R.  This  short-sighted  improvement  in  power  factor 
would  merely  result  in  an  increased  PR  loss  with  lowered  efficiency 
and  horsepower  capacity. 

The  electrical  action  in  a  plain  a.c.  series  motor  can  be  readily 
shown  by  vector  diagram,  as  in  Fig.  146.  In  the  field  we  have  a 
large  inductance  reaction  OY  and  a  small  resistance  reaction  YRt 


COMMUTATING   ALTERNATING-CURRENT  MOTORS. 


247 


the  resulting  voltage  across  the  field  terminals  being  OR.  The 
inductance  drop  RJ  in  the  armature  is  relatively  small,  and  not  very 
much  greater  than  its  resistance  drop  JD.  The  vector  RD  repre- 
sents the  voltage  required  to  overcome  the  impedance  reaction  of 
the  armature  winding,  and  OD  measures  the  voltage  balancing  the 


Field    Armature 


27TfLfl 


„     Current 

O  M 

FIG.    146. — VOLTAGE  DIAGRAM  OF  SERIES  MOTOR. 

joint  impedance  reaction  of  armature  and  field  windings.  This  is 
also  the  starting  voltage  of  the  motor,  since  this  reaction  represented 
by  OD  is  all  that  exists  with  the  motor  at  rest. 

A  c.e.m.f.  is  developed  in  the  armature  winding,  when  it  is 
rotated  in  the  magnetic  field.  This  c.e.m.f.  or  rotative  e.m.f.  is 
vectorially  represented  as  an  energy  component,  since  it  is  propor- 
tional to  the  power  required  to  overcome  the  rotative  iron  losses 
and  counter  torque  of  the  load.  If,  as  before  with  a  bipolar  model, 
Sa  be  the  total  number  of  inductors  around  the  armature  core,  and 
3>TO  the  maximum  value  of  the  field  flux,  the  value  of  the  rotative 
e.m.f.  is  expressed  by 

=  .707  $mr.p.m.  Sa 


60  X  io8 


(66) 


The  line  DX,  parallel  to  the  base  line  in  Fig.  146,  represents  the 
above  c.e.m.f.  and  OX  corresponds  to  the  applied  voltage,  produc- 
ing rotation  against  the  counter  torque.  The  value  of  this  applied 
voltage  is 

V  -  V(EIO,+  IRmr  +  (Ef+Ea)\  (67) 


248      ELECTRIC  MOTORS,    THEIR  ACTION  AND   CONTROL. 

In  the  case  of  a  completely  compensated  series  motor  the  value 
Ea  becomes  negligible  because  the  armature  inductive  e.m.f.  is 
neutralized,  as  explained  later.  The  ohmic  drop  IR  corresponds 
to  the  summation  of  the  lines  YR  and  JD,  which  vary  directly  with 
the  current  /  and  total  resistance  R.  Ef  and  Ea,  the  inductive  drops 
or  reactance  voltages  of  the  field  and  armature  windings  respect- 
ively, are  represented  by  the  lines  OF  and  JD.  These  vary  directly 
with  the  current  /  so  long  as  the  flux  density  is  low,  under  which 
condition  the  motor  torque  increases  as  the  square  of  current  which 
flows  in  both  armature  and  field.  The  counter  e.m.f.  (DX)  rises 
directly  with  speed  and  current,  at  moderate  flux  densities,  hence  it 
does  not  change  greatly  with  constant  impressed  voltage  OX, 
because  the  speed  diminishes  as  current  is  increased.  The  phase 
relation  exisiting  between  the  impressed  voltage  OX  and  the  current 
I  varies  with  the  motor  load,  the  power  factor  decreasing  as  the 
torque  is  augmented,  a  fact  borne  out  by  the  working  curves  of  the 
series  motor  in  Fig.  148.  Still  further  study  of  this  voltage  dia- 
gram (Fig.  146)  brings  out  the  fact  that  the  starting  voltage  of  the 
a.c.  motor  is  nearly  one-half  of  the  impressed  running  voltage, 
while  with  d.c.  machines  the  starting  voltage  is  small,  not  exceed- 
ing one-eighth  of  the  line  potential.  Under  the  same  conditions 
of  field  and  armature  resistance  and  low  current,  the  latter  would 
be  only  the  sum  of  the  energy  components  YR  and  JD,  because  the 
inductive  e.m.f. 's  OF  and  RJ  are  absent.  Another  fact  indicated 
by  the  vector  diagram  is  the  low  power  factor  of  the  motor  at 
starting,  being  the  lowest  value  over  the  entire  load  range;  this  is 
so  because  the  inductive  component  is  about  the  same  as  when  the 
motor  is  loaded,  while  the  energy  component  is  very  much  smaller. 

The  curves  representing  the  performance  of  a.c.  series  motors 
can  be  approximately  determined  by  means  of  a  circle  diagram. 
This  method  presupposes  constant  permeability  of  the  magnetic 
circuit,  and  hence  unvarying  reactance  of  the  motor  windings,  even 
though  the  load  current  be  considerably  varied.  This  condition 
maintains  only  approximately  in  practice,  consequently  results 
obtained  from  the  circle  diagram  are  not  rigorously  correct ;  never- 
theless, the  speed  and  power  factor  values  derived  therefrom  agree 
very  closely  with  test  results.  The  main  difficulty  with  this  method 
is  the  fact  that  the  torque  values  thus  obtained  are  founded  upon 
the  assumption  that  the  motor  torque  varies  as  the  square  of  the 


COMMUTATING  ALTERNATING-CURRENT  MOTORS.         249 

current,  whereas,  in  fact,  this  relation  exists  only  for  the  total  torque 
developed  and  with  the  magnetic  circuit  operated  well  below  satu- 
ration. An  approximation  which  gives  the  effective  torque  at  differ- 
ent current  values  with  reasonable  accuracy  will  be  given  later. 

The  circle  diagram  is  directly  developed  from  the  vectorial  voltage 
relations  shown  in  Fig.  146,  because  if  the  impedance  of  the  motor 
windings  is  constant  and  the  impressed  voltage  of  fixed  value,  the 
variations  of  current  and  power  factor  are  such  that  the  extremity 
of  the  current  vector  moves  in  the  arc  of  a  circle. 

Construction  of  Circle  Diagram.  —  Lay  off  the  line  OX,  repre- 
senting the  vector  position  of  the  line  voltage  (Fig.  147),  then  at  an 
angle  <j>s  lay  off  to  scale  the  short-circuit  current  OS.  This  short 
circuit  current  is  taken  at  full  line  voltage,  and  its  phase  angle 


iwatt  meter  reading       ,       XT  , 

cos—1  -  -  -  p  =  $5.    Next   apply  a  brake  to  motor 
volts  X  amperes 

shaft  or  driving  wheel  and  allow  the  armature  to  rotate,  adjust  the 
brake  so  that  the  machine  draws  a  certain  current,  say  about  one- 
third  of  the  short-circuit  value,  note  volts,  amperes,  watts  input, 
speed  and  torque,  calculate  speed  in  miles  per  hour,  tractive  effort, 
horsepower  output,  and  efficiency  at  this  selected  load.  Lay  off 
this  last  current  OI  to  scale  and  in  its  determined  phase  position 
with  respect  to  line  voltage.  The  locus  of  the  current  drawn  by 
the  motor  under  different  loadings  at  a  fixed  voltage  and  frequency 
is  then  the  arc  of  a  circle  drawn  through  the  points  O,  7,  and  S. 
The  power  factor  of  any  current  can  be  determined  by  project- 
ing the  intersection  of  its  vector  with  the  circle  KNC  across  to 
the  power  factor  scale  upon  OX.  The  speed  corresponding  to  any 
load  current  O7,  for  example,  is  also  determinable  from  the  circle 
diagram,  as  follows:  Draw  a  line  through  S  parallel  to  OX,  then 
continue  the  current  vector  OI  until  it  intersects  this  line  at  P\ 
the  distance  between  points  S  and  P  is  proportional  to  the  motor 
speed  existing  when  the  current  is  OI  at  voltage  OX.  This  rela- 
tion is  true  by  construction,  because  OI  is  at  constant  impedance 
proportional  to  the  impedance  drop  and  OS  to  the  line  voltage; 
consequently  IS  is  proportional  to  the  c.e.m.f.  due  to  rotation  and 
therefore  to  speed  of  rotation  itself.  Comparing  the  triangles  OIS 
and  OSP,  it  is  seen  that  they  are  similar  because  the  three  included 
angles  of  one  are  equal  to  those  of  the  other;  accordingly  SP  is  pro- 
portional to  75,  or  to  the  motor  speed  as  above  stated.  If  line  SP 


250      ELECTRIC  MOTORS,    THEIR  ACTION  AND    CONTROL. 


34 


FIG.  147. —CIRCLE  DIAGRAM  FOR  A.C.  SERIES  MOTOR. 


COMMUTATING   ALTERNATING-CURRENT   MOTORS. 


251 


be  divided  to  scale,  the  speed  at  any  assumed  current  can  be  read 
off  directly  by  continuing  its  vector  to  intersect  SP  or  its  prolon- 
gation. The  torque  of  the  motor  can  be  calculated  with  an  error 
not  exceeding  a  few  per  cent,  by  the  relation  T  =  k$>I.  Relative 
values  of  the  field  flux  <l>  existing  at  different  current  values  are  deter- 
mined from  the  e.m.f.  of  rotation,  which  in  the  circle  diagram  is 

6000 


4500 


4000 


Diam.  Wheels  38 
Gear  Ratio  3.2£ 


100 


200 


600 


700 


900        1000 


300         400          500 
Amperes 

FIG.   148.  —  CHARACTERISTIC  CURVES  25-CYCLE,   25O-VOLT, 
I50-H.P.  SERIES  MOTOR. 

proportional  to  the  line  IS  for  the  current  O/,  and  for  any  other 
current  proportional  to  the  corresponding  line.  Naturally,  to 
obtain  the  true  ratio  existing  between  the  different  rotational  e.m.f.'s 
and  therefore  between  corresponding  values  of  field  flux,  these 
e.m.f.'s  must  be  reduced  to  a  common  speed  basis.  The  torque  7\ 
with  any  current  1^  is  then  determined  from  that  exerted  with  the 
test  current  /  flowing,  by  the  relation: 

T,  =  r(«Vi  +  */).  (68) 

The  results  obtained  from  the  circle  diagram  in  Fig.  147  are 
compared  in  the  following  table,  with  values  taken  from  test  curves 


252      ELECTRIC   MOTORS,    THEIR   ACTION  AND   CONTROL. 


of  Fig.  148.     These  characteristic  curves  are  for  a  i5o-horsepower, 
2 50- volt,  2  5 -cycle  compensated-series  motor. 

COMPARISON  OF  RESULTS  OBTAINED  BY  TEST  AND  FROM  CIRCLE 

DIAGRAM. 

150-h.p.,  25-cycle,  250-volt  Compensated  Series  Motor. 


Power  Factor. 

Speed  in  M.p.h. 

K.V.A. 

H.P.  Input. 

Amperes. 

Test. 

Diagram. 

Test. 

Diagram. 

Input. 

Test. 

Diagram. 

400 

.90 

.90 

33.5 

32.7 

100 

121 

121 

500 

.86 

.86 

27.5 

27.5 

125 

144 

144 

600 

.83 

.83 

23.2 

23.4 

150 

167 

167 

700 

.79 

.80 

20.0 

20.0 

175 

188 

188 

800 

.75 

.75 

17.5 

17.0 

200 

201 

201 

900 

.71 

.70 

15.2 

14.5 

225 

214 

212. 

1000 

.67 

.66 

13.2 

12.2 

250 

224 

221. 

Amps. 

400 
500 
600 
700 
800 
900 
1000 

Rotational 
C.E.M.F. 

Torque 
Ratio 

*!/!+*/ 

Pounds, 
Tractive  Effort. 

Horsepower 
Output. 

Per  cent 
Efficiency. 

Diag. 

'  At 
Common 
Speed. 

Test. 

Diag. 

Test. 

Diag. 

Test. 

Diag. 

Volts. 
216 
207 
195 
182 
167 
153 
138 

Volts. 
181 
207 
227 
250 
273 
290 
313 

.70 
1.00 
1.32 
1.71 
2.14 
2.52 
3.02 

1175 
1750 
2300 
2975 
3650 
4300 
5050 

1210 
1750 
2300 
3000 
3740 
4410 
5300 

108 
128 
142 
150 
170 
175 
178 

105 
128 
143 
158 
170 
172 
174 

89.4 

89 
85 
84.5 
84.5 
81.6 
78 

89 
89 

85 
85 
84.5 

82 
78 

The  agreement  between  test  and  calculated  results  is  reasonably 
close.  The  differences  existing  may  be  charged  to  two  facts,  namely, 
the  basic  assumption  in  the  construction  of  the  diagram  that  the 
winding  impedance  remains  constant,  which  is  not  strictly  true,  and 
again  to  the  difficulty  of  making  very  close  linear  measurements  in 
a  small  diagram. 

An  examination  of  the  working  curves  (Fig.  148)  of  the  a.c.  series 
motor  indicates  that  the  speed-  and  torque-current  characteristics 
are  very  similar  to  those  of  the  corresponding  d.c.  machine,  but 
owing  to  the  lower  flux  densities  of  the  former,  the  torque  increases 
more  nearly  as  the  square  of  the  current  throughout  the  load  range. 


COMMUTATING   ALTERNATING-CURRENT   MOTORS.         253 


For  the  same  reason  its  speed  does  not  tend  to  become  so  nearly 
constant  at  heavier  loads.  The  motor  speed  at  fractional  voltages 
can  be  determined  from  the  circle  diagram,  or  by  the  relation  exist- 
ing between  the  rotational  e.m.f's. 

£2rot=  £2line    +  /Z2  -  2  Eline  IZ  COS  (&  -  <£) .  (69) 

In  this  expression  /  is  the  armature  current  at  which  the  value  of 
Erot  is  desired;  Z  the  impedance  of  the  windings,  fa  the  phase 
angle  of  the  short-circuit  current,  (f>  the  phase  angle  of  the  current  /, 
and  Eline  is  the  value  of  the  line  voltage  at  which  the  speed  is  to  be 
calculated. 

The  losses  in  an  a.c.  series  motor  may  be  divided  into  two  classes, 
namely,  those  which  also  occur  in  d.c.  machines,  and  those  peculiar 
to  machines  having  magnetic  fields  set  up  by  alternating  currents. 
Since  these  latter  motors  operate  at  a  lower  voltage  than  the  corre- 
sponding d.c.  machines,  the  current  required  for  a  given  power 
is  greater,  hence  the  copper  losses  will  be  more,  or  the  amount  of 
copper  required  in  the  windings  will  be  larger.  As  a  matter  of 
fact,  the  total  losses  in  an  a.c.  series  motor  are  about  twice  those 
occurring  in  a  corresponding  d.c.  motor.  The  various  losses  of 
the  a.c.  motor  may  be  conveniently  arranged  as  shown  in  the  fol- 
lowing chart.* 

[Useful 
j  Mechanical 

Mechanically    [D.C.  Iron  Losses   lPower 
Supplied  <  Friction  Losses 

Losses  [Windage  Losses     [Losses  common 

to  A.C.  and 
D.C. 


Total 

Total 
Electrical  ^ 

Mechanical     • 
Power 

Power 

Supplied 

Electrically      1 

Supplied         \ 

Losses 

[Ordinary  PR  Losses 
•I  Transformer  PR  Losses 
[A.C.  Iron  Losses 


(Special  A.C. 
ILosses 


Total 
Losses 


Compensation  of  Armature  Reaction  and  Inductance.  —  It  has 

already  been  shown  (p.  246)  that  to  improve  the  power  factor  of  an 
a.c.  series  motor,  the  practice  is  to  weaken  the  field  and  strengthen 
the  armature  by  decreasing  the  turns  of  the  former  and  increasing 
those  of  the  latter.  This  change  in  design  tends,  however,  to  exag- 
gerate armature  reaction  as  well  as  commutation  difficulties.  These 
two  troubles  can  to  a  very  marked  extent  be  reduced  or  even  elim- 
inated by  the  introduction  of  a  compensating  m.m.f.  in  substantially 

*  Electric  Club  Journal.  Vol.  I,  1904,  p.  16. 


254      ELECTRIC   MOTORS,    THEIR   ACTION   AND   CONTROL. 


the  same  manner  as  employed  in  the  case  of  d.c.  adjustable  speed 
shunt  motors  of  the  Thompson-Ryan  design  pp.  49, 55 .  This  method 
of  preventing  the  field  distortion  by  the  armature  m.m.f.  and  reduc- 
ing the  armature  inductance  is  to  surround  the  revolving  armature 
with  a  fixed  winding  placed  in  slots  cut  in  the  polar  faces  if  salient 


FIG.  149.  —  SERIES  MOTOR  WITH  COMPENSATING  WINDING 
(CONDUCTIVE  COMPENSATION)  . 

poles  are  employed,  or  if  an  induction  motor  stator  frame  is  used 
the  compensating  winding  is  displaced  90  magnetic  degrees  or  half 
a  pole  pitch  from  the  field  winding.  The  compensating  coils  carry 
a  current  equal  in  m.m.f.  and  opposite  in  phase  to  the  current  in 
the  armature,  and  this  current  may  be  obtained  either  conductively 
by  connecting  the  balancing  winding  directly  in  series  with  the  field 


FIG.  150. SERIES  MOTOR  WITH  COMPENSATING  TRANSFORMER 

(INDUCTIVE  COMPENSATION)  , 

and  armature  windings,  as  in  Fig.  149,  or  inductively  by  using  the 
stationary  winding  as  the  short-circuited  secondary  of  a  trans- 
former, of  which  the  armature  is  the  primary,  as  in  Fig.  150.  It  is 
found  that  the  best  effects  are  produced  when  the  balancing  of  the 
armature  reaction  is  complete.  The  conductive  method  is  the  more 
desirable  when  the  motor  is  to  be  operated  on  mixed  service,  that  is, 
partly  on  a.c.  circuits  and  partly  on  d.c.  circuits. 

Methods  of  Control  of  A.C.  Series  Motors.  — The  series  motor 
may  be  controlled  by  means  of  a  rheostat  in  series  with  it,  an  auto- 
transformer  or  an  induction  regulator.  With  external  resistance 


COMMUTATING   ALTERNATING-CURRENT  MOTORS. 


255 


the  efficiency  of  the  system  is  low,  as  in  the  case  of  rheostatic 
control  with  d.c.  machines.  When  the  auto-transformer  is  em- 
ployed, the  line  is  bridged  by  a  single  coil  transformer  provided 
with  taps,  so  that  various  voltages  can  be  applied  to  the  motor 
circuit,  and  low  voltages  for  starting  can  be  obtained  without  the 


FIG.  151. CONNECTIONS  FOR  OPERATING  SERIES  MOTOR  ON  MIXED  SERVICE. 

large  losses  involved  in  resistance  control.  The  speeding  up  of 
the  motor  is  accomplished  by  including  more  and  more  sections  of 
the  auto-transformer  between  the  motor  terminals. 

The  trolley  voltage  usually  employed  in  a.c.  traction  work  is 
11,000  volts  or  thereabouts,  while  the  motors  are  designed  to  oper- 
ate at  250  volts  or  less.  The  line  from  the  trolley  to  the  ground 
passes  through  an  auto-transformer  designed  with  taps  so  as  to 
give  an  adjustable  secondary  pressure  up  to  500  volts,  sufficient  to 


256      ELECTRIC   MOTORS,    THEIR   ACTION   AND    CONTROL. 

operate  two  motors  in  series.  The  general  scheme  of  connecting 
a.c.  series  motors,  with  auto-transformer  control,  for  traction  service 
is  shown  in  Fig.  151.  Since  these  motors  are  rated  at  250  volts  each, 
they  are  connected  in  series-parallel  groups,  two  motors  being  per- 
manently connected  in  series  so  as  to  fit  them  also  for  d.c.  operation. 
The  switch  S,  automatically  operated,  cuts  out  the  auto-transformer 
when  the  alternating  current  fails,  and  inserts  the  rheostatic  or  series- 
parallel  control  of  the  two  groups  necessary  for  d.  c.  service.  Switches 
aa  and  cc  are  open,  while  b  and  d  are  closed  for  series  connection  of 
motors  —  the  converse  is  the  order  for  parallel  service.  The 
switch  RS  reverses  the  current  in  the  field  coils  and  thus  the  direction 
of  rotation.  The  auto-transformer  method  of  controlling  the  speed 
of  a.c.  series  motors  corresponds  to  the  multiple  voltage  (p.  74) 
and  motor-generator  systems  for  d.c.  motors,  because  they  all 
supply  an  adjustable  voltage  corresponding  to  the  speed  desired. 
The  a.c.  means  are  much  simpler,  however;  in  fact  the  facility  of 
transforming  voltage  is  the  great  advantage  of  the  a.  c.  control. 

The  Repulsion  Motor.  — As  stated  in  Chapter  XII,  the  physical 
phenomenon  upon  which  the  operation  of  this  motor  largely  depends 


FIG.  152.  —  REPULSION  OF  SINGLE  COIL. 

was  discovered  by  Prof.  Elihu  Thomson  in  1887,  and  he  applied  it 
the  same  year  to  the  development  of  an  experimental  motor.* 
The  production  of  the  repulsion  phenomenon  is  as  follows:  If 
a  closed  coil  of  wire  be  suspended  or  pivoted  near  the  pole  of  an 
alternating  current  magnet  in  such  a  manner  that  lines  of  force 
from  the  later  pass  through  the  former  as  represented  in  Fig.  152, 
an  alternating  e.m.f.  will  be  induced  in  the  coil.  This  secondary 
e.m.f.  will  be  90  degrees  later  in  phase  than  the  inducing  flux,  and 

*  Transactions  A.  I.  E.  E.,  Vol.  IV,  1887,  p.  160.     *  U.  S.  Patent  No.  263,185  of  1887. 


COM  MUTATING   ALTERNATING-CURRENT   MOTORS.         257 

if  the  coil  contains  no  inductance,  the  secondary  current  will  also 
be  in  quadrature  with  the  primary  flux.  Under  such  conditions 
there  will  be  no  development  of  torque  by  the  mutual  action  of  the 
magnetic  field  and  current  as  explained  on  page  239.  Practically, 
however,  every  coil  of  wire  contains  some  inductance,  so  that  the 
secondary  current  lags  more  or  less  with  respect  to  the  secondary 
e.m.f.,  being  therefore  more  than  90  degrees  later  than  the  primary 
flux,  with  the  result  that  the  cosine  of  this  phase  relation  becomes  a 
negative  quantity.  This  means  that  the  coil  is  repelled  by  the  field. 
The  maximum  repulsion  occurs  theoretically  if  primary  flux  and 
secondary  current  differ  in  phase  by  180  degrees,  but  even  to  ap- 
proximate this  value  requires  the  coil  reactance  (<uL)  to  be  very 
large  with  respect  to  its  resistance.  This  condition  implies  an 
extremely  small  current,  so  practically  the  maximum  repulsion 
occurs  when  the  coil  has  such  impedance  that  the  secondary  current 
lags  about  45  degrees  with  respect  to  the  e.m.f. 

If  the  movable  coil  above  considered  be  pivoted  in  the  magnetic 
field,  the  only  way  in  which  the  negative  torque  or  repulsion  can  act 
is  by  turning  this  coil  on  its  axis,  until  such  position  is  reached  that 
no  lines  of  force  pass  through  it,  or  in  other  words,  it  will  turn  until 
it  assumes  a  position  parallel  to  the  lines  of  force.  A  coil  perpen- 
dicular to  the  flux  may  rotate  in  either  direction,  hence  it  must  be 
placed  obliquely  with  respect  to  the  flux,  to  compel  rotation  in  a 
definite  direction,  and  if  the  inertia  of  the  coil  be  sufficient  to  carry 
it  beyond  the  dead  center,  continuous  motion  will  be  developed. 

The  elementary  repulsion  motor  devised  by  Professor  Thomson 
is  diagrammatically  illustrated  in  Fig.  153.  The  magnetic  circuit 
was  completely  laminated  and  the  armature  winding  was  of  the  open 
coil  type,  the  terminals  of  each  coil  being  connected  to  diametrically 
opposite  commutator  bars.  The  field  winding  was  connected 
directly  across  the  line,  and  the  armature  short-circuited  by  means 
of  diametrically  opposite  brushes  connected  by  a  copper  lead. 
With  these  brushes  placed  so  that  they  short-circuit  the  armature 
coils  at  an  oblique  angle  to  the  flux  direction,  torque  and  rota- 
tion are  set  up  as  for  the  single  coil  already  considered  and  are  con- 
tinued through  the  successive  action  of  the  different  coils.  The 
limitation  of  this  early  type  of  repulsion  motor  is  the  fact  that 
the  effective  torque  developed  at  any  moment  is  due  only  to  a  single 
armature  coil,  since  no  current  exists  in  the  others  whose  circuits 


258      ELECTRIC  MOTORS,    THEIR   ACTION   AND   CONTROL. 


are  open.  Hence  to  develop  any  considerable  power,  the  current 
in  the  short-circuited  coil  must  necessarily  be  high,  and  the  opening 
of  this  circuit  as  the  corresponding  commutator  bars  pass  out  of 
contact  with  the  brushes  causes  excessive  sparking.  Professors 


FieldJErame 


Field 
Coils 


FIG.   153.         EARLY  THOMSON  REPULSION  MOTOR. 

Anthony,  Ryan  and  Jackson  appreciated  the  seriousness  of  this 
defect,  and  in  1888  suggested  the  use  of  a  closed  coil  armature 
winding  in  place  of  the  open  coil  type.*  This  resulted  in  a 
greatly  increased  power  for  a  given  weight,  because  the  effective 
turns  on  the  armature  were  augmented  and  a  given  current  pro- 
duced more  torque,  or  a  smaller  current  produced  the  same  torque 
without  as  much  sparking.  On  the  other  hand,  sparking  with  this 
type  of  armature  is  due  not  only  to  reversal  of  current  in  the  coil 
short-circuited  by  the  brush,  as  in  d.c.  machines,  but  also  to  trans- 
former action,  as  already  explained  with  reference  to  the  series  a.c. 
motor  (p.  242). 

Sparking  in  the  brushes  in  the  more  modern  designs  is  reduced  by 
compensation,  as  in  the  series  motor;  by  use  of  a  distributed  field 
winding;  high  brush  contact  resistance;  prevention  leads,  etc. 
With  the  simple  form  of  repulsion  motor  indicated  in  Fig.  154,  the 
field  winding  is  directly  across  the  line  and  there  is  no  electrical 
connection  between  the  armature  and  field  or  supply  circuit,  re- 
sembling in  this  respect  the  transformer  with  a  leaky  magnetic 
circuit  and  movable  secondary  winding. 

*  Trans.  A.  I.  E.  E.,  Vol.  XXIII,  1904,  p.  77. 

*  U.  S.  Patent  389,352,  September,  1888. 


COMMUTATING  ALTERNATING-CURRENT  MOTORS.         259 

The  flux  impressed  on  the  armature  core  by  the  field,  and  repre- 
sented in  Fig.  155  by  the  vector  OR,  may  be  resolved  into  two 
components,  the  first  being  OB  along  the  line  of  commutation  of 
the  armature  winding  and  the  second  OA  perpendicular  thereto. 


FIG.  154.  —  CONNECTIONS  OF  SIMPLE  REPULSION  MOTOR. 

Currents  are  developed  in  the  armature  winding  by  two  independ- 
ent actions,  namely,  transformer  and  rotational  induction.  The 
component  OB  is  that  which  produces  current  in  the  armature 
winding  by  transformer  effects,  while  OA  is  that  producing  current 


FIG.  155.  —  COMPONENTS  OF  REPULSION  MOTOR  FIELD  FLUX. 

in  the  secondary  winding  by  rotation.  These  two  independently 
produced  armature  currents  are  combined  to  give  the  total  armature 
current  actually  existing. 

The  line  voltage  applied  across  the  field  terminal  may  be  regarded 
as  being  made  up  of  three  components,  namely:  First,  the  com- 
ponent required  to  overcome  the  resistance  drop  in  the  field  winding; 


260      ELECTRIC  MOTORS,    THEIR   ACTION  AND   CONTROL. 

second,  that  required  to  overcome  the  equivalent  reactance  of  the 
primary  winding;  and  third  that  needed  to  overcome  the  e.m.f. 
induced  in  the  field  winding  by  the  rotation  of  the  armature. 

By  considering  these  three  components,  a  circle  diagram  some- 
what similar  to  that  of  the  series  motor  can  be  developed.  The 
torque  of  the  repulsion  motor  cannot  be  assumed  to  vary  as  the 
square  of  the  current,  since  the  phase  relation  between  stator  and 
rotor  currents,  as  well  as  the  brush  position,  must  be  considered. 

An  excellent  performance  diagram  of  the  repulsion  motor  was 
given  by  Mr.  Osnosinthe  "  Electrotechnische  Zeitschrift"  for  Oct.  29. 
1903,  p.  905.  This  diagram  is  shown  in  Fig.  156,  and  its  construc- 


B  D    r  w 

c- 

FIG.  156.  —  THE  OSNOS  CIRCLE  DIAGRAM  OF  THE  REPULSION  MOTOR. 

lion  is  as  follows:  OE  represents  the  direction  of  the  impressed 
voltage,  O/j  the  primary  current  with  the  rotor  locked,  drawn  at 
an  angle  EOI^  corresponding  to  its  phase  displacement.  Olf  is 
the  current  with  the  rotor  revolving  without  load  and  EOIf  is  its 
corresponding  phase  angle.  Draw  an  arc  of  a  circle  through  OI/IV 
the  point  C  being  the  center  of  the  circle.  This  is  the  circle  of 
current  input,  and  the  angle  EOI  corresponds  to  the  phase  angle  of 
any  particular  primary  current.  The  ordinate  IP  is  the  working 
or  energy  component  of  the  current  /  and  is  proportional  to  the 
input.  Describe  the  circle  BI^D,  which  has  its  center  on  OB 
perpendicular  to  OE.  Then  on  OB  as  a  diameter  describe  a 
second  semicircle  OI/D.  The  first  of  these  semicircles  or  BIeD 
is  the  circle  of  speed  and  the  second  circle  OI0D  is  the  torque  circle. 
Thus  for  a  load  requiring  a  current  /,  the  speed  is  represented  by 
the  ratio  IP/PD,  and  the  torque  by  the  product  OI  X  IN.  The 
line  ID  represents  the  secondary  current  in  phase  and  magnitude 


COMMUTATING   ALTERNATING-CURRENT   MOTORS. 


261 


(reduced  to  primary  equivalents).  The  angle  d  is  the  difference 
in  phase  between  corresponding  primary  and  secondary  currents. 
This  diagram  takes  into  account  the  copper  losses  as  well  as  leakage 
effects,  but  does  not  include  the  windage,  friction  or  iron  losses, 
which  must  be  allowed  for,  either  by  addition  to  the  input  or  sub- 
traction from  the  output. 

The  speed  regulation  of  the  repulsion  motor  may  be  altered  by 
simply  shifting  the  brushes.  This  type  of  motor,  however,  is  very 
sensitive  to  comparatively  slight  change  of  the  brush  position,  hence 
extreme  care  should  be  taken  in  attempting  thus  to  vary  the  speed 
characteristics. 

The  direction  of  rotation  of  the  repulsion  motor  may  be  reversed, 
either  by  shifting  the  brushes  over  to  the  other  side  of  the  neutral 
line  of  the  field  flux,  that  is  from  AB  to  CD  in  Fig.  154,  or  by 
shifting  the  primary  connections  by  90  degrees  magnetically  when 
a  distributed  closed  coil  stator  winding  is  employed. 

The  Compensated  Repulsion  Motor  *  is  a  development  of  the 
preceding  motor  and  was  designed  with  the  object  of  overcoming 
field  distortion,  and  to  increase  the  power  factor  of  the  machine. 


FIG.  157.  —  CONNECTIONS  OF  COMPENSATED  REPULSION  MOTOR. 


The  diagrammatic  connections  of  the  simplest  form  of  this 
modification  are  shown  in  Fig.  157.  At  first  sight,  this  motor 
does  not  differ  much  from  the  ordinary  series  machine,  but  the 
presence  of  brushes  B  and  B  considerably  modifies  the  action.  One 
effect  is  largely  to  neutralize  the  self-inductance  of  the  field  winding, 
since  the  current  flowing  in  the  armature  across  these  brushes  acts 
as  the  current  of  a  short-circuited  secondary,  of  which  the  field  wind- 
ing is  the  primary.  The  field  winding,  therefore,  acts  as  a  trans- 
former coil;  on  the  other  hand,  it  does  not  supply  the  entire  magnetic 

*  Transactions,  1904,  International  Elect.  Congress,  Vol.  Ill,  pp.  129-185. 


262      ELECTRIC   MOTORS,    THEIR   ACTION   AND    CONTROL. 

field  necessary  for  the  production  of  the  turning  effort.  This  latter 
field  is  now  mainly  supplied  by  that  component  of  the  current  which 
passes  through  the  armature  at  brushes  bb.  The  current  flowing 
between  bb  is  variously  known  as  the  exciting  or  compensating 
current,  while  that  developed  between  brushes  BB  is  called  the 
short-circuit  current.  This  type  of  motor  is  characterized  by  high 
power  factor  at  speeds  above  synchronism,  but  at  low  speed  its  power 
factor  is  less  than  with  the  a.c.  series  motor,  while  at  all  speed 
points  its  torque  per  ampere  is  not  as  high. 

A  further  criticism  of  this  construction  is  the  fact  that  the  greatest 
advantage  of  the  repulsion  motor  —  its  connection  directly  to  high 
tension  lines  —  is  no  longer  practicable,  because  now  the  revolving 
member  is  also  in  the  main  circuit  so  that  the  necessary  insulation  is 
difficult.  This  bad  feature  of  the  compensated  motor  is  avoided 
by  the  Winter-Eichberg  modification  shown  in  Fig.  158.  In  this 


vs 


FIG.  158.  —  CONNECTIONS  OF  WINTER-EICHBERG  COMPENSATED 
REPULSION  MOTOR. 

design  the  armature  exciting  current,  flowing  between  brushes  bb, 
instead  of  being  supplied  directly  to  the  armature  from  the  high 
tension  lines,  is  obtained  from  the  secondary  of  a  transformer  whose 
primary  is  in  series  with  the  stator  and  high  tension  circuit.  Vari- 
able speed  is  obtainable  by  variation  of  the  voltage  supplied  by 
this  secondary,  which  is  provided  with  taps,  as  shown. 

Even  though  the  repulsion  motor,  in  its  various  forms,  possesses 
the  majority  of  the  desirable  features  of  the  a.c.  series  motor,  with 
the  possibility  of  even  better  power  factor  at  higher  frequencies, 
combined  with  high  voltage  supply  to  which  the  armature  is  not 
subjected,  nevertheless,  it  has  not  been  as  favorably  received  in 


COMMUTATING   ALTERNATING-CURRENT   MOTORS.         263 

this  country  as  the  series  machine,  on  account  of  the  following 
features : 

1.  Noisy  in  starting  up. 

2.  Lower  starting  torque  per  ampere,  on  account  of  the  "blow- 
ing out"  of  the  primary  flux,  due  to  secondary  flux  of  the  armature, 
as  well  as  the  difference  in  phase. 

3.  Greater  tendency  to  spark  at  the  brushes,  due  to  excessive 
transformer  currents,  upon  which  its  action  largely  depends. 

4.  With  compensated  motors,  though  sparking  is  much  less,  the 
motor  is  provided  with  twice  as  many  sets  of  brushes,  which  are 
always  a  source  of  weakness  and  trouble,  especially  in  traction 
service. 

5.  Reversal  of  direction  of  rotation  not  as  convenient  as  with 
the  series  motor. 

6.  Shifting  from  a.c.  to  d.c.  and  back  again  not  as  easy  as 
with  series  motor,  because  of  the  extra  short-circuiting  brushes, 
commutation  devices,  etc. 

COMPARISON  OF  D.C.  AND  A.C.  COMMUTATOR  MOTORS. 

With  d.c.  machines,  each  circuit  or  portion  thereof,  whether  in 
series  or  shunt,  must  obtain  its  current  by  electrical  connection  to 
the  supply  conductors.  On  the  other  hand,  a.c.  circuits  may  be 
supplied  inductively  as  well  as  conductively.  For  example,  any 
d.c.  motor  must  have  its  field  and  armature  windings,  also  its  com- 
pensating winding  to  neutralize  armature  reaction  (if  provided 
therewith),  all  connected  to  the  supply  circuit.  The  windings  of 
an  a.c.  motor,  however,  may  be  energized  in  four  different  ways: 
first,  by  electrical  connection  to  and  conduction  from  the  supply; 
second,  by  induction  through  a  transformer;  third,  by  induction 
in  the  winding  short-circuited  upon  itself;  and  fourth,  by  induction 
to  another  one  of  the  three  circuits  to  which  the  given  winding  is 
connected  as  a  tertiary  circuit. 

The  Winter-Eichberg  motor  in  Fig.  158  has  its  field  winding 
directly  connected  to  the  supply  circuit;  the  armature  receives 
current  by  the  brushes  b  and  b  from  the  secondary  of  the  transformer 
PS  and  at  the  same  time  the  brushes  B  and  B  are  connected  by 
very  low  resistance  so  as  to  short-circuit  the  armature.  Thus  the 
first  three  of  the  above  arrangements  are  in  most  a.c.  commutator 
machines  present  in  this  one  machine.  The  field  winding  is  con- 


264      ELECTRIC   MOTORS,    THEIR   ACTION   AND    CONTROL. 

nected  directly  to  the  supply  conductors  because  it  is  more  easily 
insulated  for  the  high  voltage  which  they  usually  carry.  The  arma- 
ture, on  account  of  its  construction  and  motion,  is  more  difficult  to 
insulate,  and  for  that  reason  is  often  connected  as  a  secondary  cir- 
cuit, the  voltage  of  which  may  be  made  as  low  as  desired.  This 
arrangement  is  characteristic  of  the  repulsion  motor  and  constitutes 
one  of  its  most  prominent  advantages.  The  possibility  of  ener- 
gizing any  one  of  the  three  windings  of  an  a.c.  commutator  motor 
in  any  one  of  the  ways  specified  above  affords  opportunity  for  making 
many  different  combinations,  but  this  does  not  mean  that  all  of  them 
are  practically  advantageous. 

Any  a.c.  machine  may  be  protected  from  high-voltage  by  supply- 
ing it  through  a  transformer.  Such  protection  is  not  absolute  in 
the  case  of  an  auto-transformer,  but  the  potential  at  the  motor  may 
be  made  as  low  as  desired.  This  arrangement  at  the  same  time 
enables  variable  voltage  to  be  easily  obtained  for  speed  control,  as 
explained  in  connection  with  Figs.  151  and  158.  The  facility  of 
transformation  and  variation  of  voltage  constitute  the  advantages 
of  a.c.  compared  with  d.c.  commutator  machines.  On  the  other 
hand,  the  low  flux  densities  and  power  factor,  also  sparking  diffi- 
culties of  the  former,  render  them  less  powerful  and  more  trouble- 
some than  the  latter.  In  other  words,  they  cost  more  and  develop 
less  power  pound  for  pound,  and  at  the  same  time  are  less  satis- 
factory in  operation. 

For  further  information  on  commutating  a.c.  motors,  the  reader  is  referred  to: 

ALTERNATING-CURRENT  ELECT.  RAILWAY.  G.  B.  Lamme.  Trans.  A.  I.  E.  E.,  Vol. 
XX,  1902,  p.  15. 

ALTERNATING-CURRENT  MOTORS.     A.  S.  McAllister.     New  York,  1909. 

ALTERNATING-CURRENT  RAILWAY  MOTORS.  W.  I.  Slichter,  Dr.  C.  P.  Steinmetz 
and  W.  A.  Blank.  Trans.  A.  I.  E.  E.,  Vol.  XXIII,  1904,  pp.  i,  9  and  83. 

ELECT.  TRACTION.     Wilson  and  Lydall.     Vol.  2.     London,  1907. 

HISTORY  AND  DEVELOPMENT  or  SINGLE-PHASE  COMMUTATOR  MOTORS.  Feldmann, 
Haga,  and  Noome.  Der  Ingenieur,  March,  1909. 

SINGLE-PHASE  COMMUTATOR  MOTORS.     F.  Punga,  R.  F.  Looser.     1906. 

SINGLE-PHASE  COMMUTATOR  MOTORS.  J.  Fischer-Hinnen.  Elect.,  London,  Vol. 
63,  1909. 

SINGLE-PHASE  COMMUTATOR  MOTORS.  M.  Deri,  M.  Latour,  O.  Bragstad,  E.  Dan- 
ielson.  Inter.  Elect.  Cong.,  St.  Louis,  1904,  Vol.  Ill,  pp.  129-184. 

SINGLE-PHASE  R.  R.  MOTORS  AND  CONTROL.  T.  H.  Schoepf.  Jour.  Inst.of  E.  E., 
London,  Vol.  36,  1906. 

UEBER  WECHSELSTROMME-KOMMUTATOR  MOTOREN.  M.  Osnos,  also  F.  Eichberg, 
Elecirotechnische  Zeitschrift,  Vol.  25,  Vol.  27,  Vol.  29  (1904-1908). 


CHAPTER    XIX. 

SERVICE   CONDITIONS   AND    APPLICATIONS   OP   ELECTRIC 

MOTORS. 

ELECTRIC  motors  employed  as  a  source  of  driving  power  must  be 
adapted  in  speed  and  torque  to  the  particular  purpose  to  which 
they  are  applied.  For  example,  if  the  speed  of  the  driven  machine 
is  required  to  be  practically  constant,  the  motor  should  also  be 
suitable  for  constant  speed  operation.  It  is  not  necessary  in  such  a 
case  for  the  motor  and  the  machine  driven  by  it  to  have  the  same 
speed,  the  conditions  being  often  fulfilled  more  conveniently  by  a 
fixed  ratio  of  speeds  obtained  through  gearing  or  belting  instead  of 
by  direct  connection.  To  secure  very  low  or  very  high  speeds  this 
arrangement  with  large  speed  ratios  usually  becomes  practically 
necessary,  as  in  the  case  of  a  triplex  plunger  pump  running  at  about 
50  r.p.m.  connected  to  a  5-horsepower  motor  whose  normal  speed 
is  about  800  r.p.m.  Another  example  is  afforded  by  the  "buzz" 
wood  planer,  the  cutting  cylinder  of  which  rotates  at  about 
4000  r.p.m.  driven  by  a  3-horsepower  motor  at  say  1000  r.p.m. 

If  the  driven  machine  or  car  is  to  run  at  variable  velocity,  then 
the  speed  of  the  motor  must  be  varied  in  the  same  proportion  with 
either  direct  connection  or  fixed  ratio  of  speeds.  It  is  possible  to 
make  use  of  mechanical  connections,  such  as  two  or  more  sets  of 
gearing  as  in  gasoline  automobiles,  to  obtain  variable  speed  ratios. 
These  may  be  employed  either  in  place  of  or  in  combination  with 
the  speed  adjustment  of  the  electric  motors,  depending  upon  cir- 
cumstances. In  many  cases,  however,  it  is  preferable  to  adjust  the 
speed  of  the  motor  electrically  rather  than  to  introduce  two  or  more 
sets  of  gears  or  other  mechanical  means  of  speed  control.  For 
instance,  a  radial  drill  costs  less  and  is  more  convenient  with  a  3 :  i 
adjustable-speed  motor  than  with  change  gear  box  and  constant- 
speed  motor.  There  are  some  cases,  especially  with  large  machines 
which  run  at  one  speed  for  several  hours,  and  only  infrequently 
at  a  different  speed.  Under  these  conditions  it  would  prob- 
ably be  as  well  to  use  a  practically  constant-speed  motor  and 

265 


266      ELECTRIC  MOTORS,    THEIR  ACTION  AND    CONTROL. 

adjust  the  speed  ratios  mechanically.  The  time  required  for  gear 
changing  is  unimportant  in  such  cases,  because  it  occurs  but 
occasionally.  In  lathes,  milling  machines  and  similar  tools  it  is 
practically  necessary  to  introduce  gearing  for  the  very  low  speeds. 
The  Speed  Classification  of  Motors  recommended  in  the  Standardi- 
zation Rules  of  the  A.  I.  E.  E.,  Transactions,  Vol.  26,  p.  1800, 
June,  1907,  is  as  follows: 

1.  Constant-speed  motors,  in  which  the  speed  is  either  constant 
or  does  not  materially  vary,  such  as  synchronous  motors,  induction 
motors  with  small  slip  and  ordinary  direct-current  shunt  motors. 

2.  Multispeed  motors  (two-speed,  three-speed,  etc.),  which  can 
be  operated  at  any  one  of  several  distinct  speeds,  these  speeds  being 
practically  independent  of  the  load,  such  as  motors  with  two  arma- 
ture windings. 

3.  Adjustable-speed  motors,  in  which  the  speed  can  be  varied 
gradually   over   a   considerable   range,   but   when   once   adjusted 
remains  practically  unaffected  by  the  load,  such  as  shunt  motors 
designed  for  a  considerable  range  of  field  variation. 

4.  Varying-speed  motors,  or  motors  in  which  the  speed  varies 
with  the  load,  decreasing  when  the  load  increases,  such  as  series 
motors. 

Classes  of  Service. — In  order  properly  to  understand  the  action 
and  speed  control  of  electric  motors  it  is  important  to  consider  at 
least  in  a  general  way  their  application  to  the  many  practical  uses 
for  which  they  are  now  employed. 

The  operating  conditions  of  almost  all  kinds  of  machinery  with 
respect  to  varying  speed,  torque  and  power  may  be  divided  into 
five  general  cases,  as  follows: 

(a)  Service   requiring   practically   constant  speed,   regardless  of 
changes  in  torque,  sudden  as  well  as  gradual. 

(b)  Service  in  which  the  torque  is  fairly  steady  or  varies  as  some 
function  of  the  speed  should  the  latter  change. 

(c)  Service  which  involves  frequent  starting  and  stopping  or  wide 
variations  in  speed  with  rapid  acceleration. 

(d)  Service  which  involves  frequent  starting  and  stopping  or  wide 
variations  in  speed  with  gradual  acceleration,  including  very  slow 
operation  or  ''inching." 

(e)  Service  in  which  the  torque  varies  regardless  of  the  speed,  or 
for  which  speed  variations  may  be  desired  irrespective  of  torque. 


APPLICATIONS  OF  ELECTRIC  MOTORS.  267 

The  first  case  (a)  includes  the  driving  of  one  or  more  machines 
by  a  single  electric  motor  running  at  practically  constant  speed. 
A  wood-working  shop  with  circular  saws,  band  saws,  planers,  etc.,  is 
a  common  and  typical  example.  The  direct-current  shunt-wound 
motor  and  the  alternating-current  induction  or  synchronous  motors 
are  applicable  to  this  purpose.  Direct-current  compound- wound 
motors  are  particularly  suitable  if  heavy  machinery  must  be  started 
from  rest  or  where  heavy  overloads,  even  momentary,  are  likely  to 
occur.  On  the  other  hand  the  speed  of  compound  motors  with 
variable  torque  is  not  so  closely  constant  as  in  the  case  of  shunt- 
machines,  but  in  many  practical  instances  the  difference  would  not 
be  objectionable.  The  torque-exerting  capabilities  of  compound 
motors  and  their  speed  characteristics  have  been  discussed  in 
Chapter  XI. 

A  single  machine  driven  by  a  motor  may  be  directly  connected 
by  coupling  their  shafts  or  by  employing  the  same  shaft  for  both, 
provided  they  are  adapted  to  run  or  can  properly  be  made  to  run 
at  the  same  speed.  Buffing  wheels,  emery  wheels  or  tool  grinders 
mounted  upon  or  directly  coupled  to  the  motor  shaft  are  prominent 
and  characteristic  examples.  If  their  speeds  are  different  they 
may  be  connected  by  belting  for  moderate  speed  ratios,  and  these 
should  not  ordinarily  exceed  4  to  i.  It  is  customary  to  use  gear- 
ing or  chain  belt  for  reducing  speed  in  higher  ratios,  especially 
for  positive  driving.  About  8  to  i  speed  ratio  is  the  practical  limit 
of  a  satisfactory  chain  drive.  On  the  other  hand,  with  sufficient 
distance  between  centers,  spur  gearing  will  give  almost  any  speed 
reduction  with  very  high  efficiency. 

The  driven  machine  may  require  constant  torque  as  well  as 
constant  speed  and  therefore  constant  power  as  in  an  ordinary 
pumping  installation.  Thus  there  would  be  nothing  to  cause 
speed  variation,  but  even  with  these  simple  conditions  d.c.  shunt 
motors  or  a.c.  induction  motors  should  be  used  because  their  speed 
is  definite  with  given  voltage  or  frequency,  a  certain  speed  being 
usually  desired,  and  the  danger  of  running  away  which  exists  with 
a  series  motor  is  avoided.  In  most  power  applications,  however, 
included  in  this  first  case  (a)  the  torque  and  power  demanded  of 
the  motor  are  variable  even  with  practically  constant  speed.  Very 
often,  indeed,  the  torque  and  power  may  be  almost  nil  at  one 
moment  and  at  rated  value  or  even  greater  a  second  or  two  later. 


268      ELECTRIC  MOTORS,    THEIR  ACTION   AND    CONTROL. 

Such  extreme  changes  occur  very  frequently  with  circular  saws, 
grindstones,  drills,  punching  presses,  shears,  buffing  wheels  and 
many  other  kinds  of  machinery.  For  extremely  or  even  moderately 
variable  torque  with  constant  speed  the  d.c.  shunt  motor  or  a.c. 
induction  or  synchronous  motors  are  especially  applicable,  also  the 
d.c.  compound  motor  for  strong  starting  torque  or  temporary  over- 
loads as  noted  above. 

A  number  of  machines  operated  by  one  motor  are  usually  driven 
through  a  line  shaft  by  pulleys  and  belting.  This  arrangement  dis- 
tributes the  power  conveniently,  enables  the  various  machines  to  be 
run  at  different  speeds  by  using  various  ratios  of  pulley  diameters  and 
readily  permits  the  starting  and  stopping  of  individual  machines  by 
clutches  or  shifting  belts.  A  group  of  wood-working  or  many  other 
kinds  of  machines  are  often  driven  by  a  single  motor  in  this  way, 
as  already  stated.  Usually  the  number  in  use  and  the  torque  de- 
manded by  each  are  varying  greatly,  at  the  same  time  approximately 
constant  speed  is  desired,  hence  the  direct-current  shunt  or  the 
alternating-current  induction  or  synchronous  motor  is  employed. 

A  refinement  of  this  problem  is  encountered  in  the  driving  of 
textile  machinery,  especially  silk  looms,  with  which  even  a  slight 
speed  variation  might  affect  the  appearance  of  the  finished  product. 
In  such  instances  the  alternating-current  induction  or  synchronous 
motors  are  generally  employed  because  the  speed  of  direct-current 
motors  varies  Considerably  with  voltage  changes  and  with  the 
variation  in  temperature  which  occurs  after  several  hours  of  opera- 
tion, as  explained  in  Chapter  III,  whereas  the  speed  of  the  alter- 
nating-current motors,  unless  the  voltage  varies  greatly,  is  dependent 
upon  the  frequency  of  the  supplied  current. 

The  second  case  (b)  covers  service  in  which  the  torque  is  fairly 
steady  or  varies  with,  but  usually  more  rapidly  than,  the  speed  if 
the  latter  changes.  This  case  includes  the  operation  of  pumps, 
fans  or  blower  equipments,  and  its  requirements  are  satisfied  by  the 
series  motor,  whose  speed  adjusts  itself  to  the  work  and  because  it 
exerts  the  maximum  torque  required  at  starting.  It  must  be, 
however,  either  geared  or  directly  connected  to  the  apparatus,  be- 
cause the  breaking  of  the  belt  or  the  sudden  removal  of  the  load 
would  cause  a  series  motor  to  race  and  become  injured  (p.  100). 
To  avoid  these  dangers  or  to  permit  the  use  of  a  clutch  which 
might  allow  a  series  motor  to  run  away,  also  because  very  widely 


APPLICATIONS  OF  ELECTRIC   MOTORS.  269 

variable  speeds  are  undesirable,  it  is  common  practice  to  employ 
heavily  compound-wound  motors  for  driving  reciprocating  pumps 
or  positive  blowers.  If  a  break  should  occur  in  the  suction  pipe  of  a 
pump  a  series  motor  is  likely  to  race,  while  a  compound  motor  would 
not  rise  in  speed  above  the  danger  limit.  The  operation  of  pumps 
by  electric  motors  is  usually  effected  by  gearing,  since  ordinary 
plunger  pumps  do  not  operate  efficiently  if  driven  in  excess  of  fifty 
strokes  per  minute,  and  to  accomplish  this  by  direct  connection 
would  demand  a  very  low  speed  and  costly  motor.  Centrifugal 
pumps  or  blowers  operating  at  high  speed  may  be  direct  driven. 

The  third  case  (c)  includes  electric  traction  and  crane  service, 
in  which  the  motor  is  frequently  started  and  stopped  and  rapidly 
accelerated '  at  starting,  adjusting  itself  automatically  to  the  load, 
slowing  down  when  heavily  loaded  .as  when  a  car  is  climbing  a 
steep  grade.  These  conditions  are  satisfied  by  series  motors  of 
either  the  direct-  or  alternating-current  types,  depending  upon  the 
current  available.  Elevator  service  is  of  this  character,  as  regards 
frequent  starting  and  stopping,  but  after  rapid  acceleration  it  calls 
for  a  speed  independent  of  the  load.  Hence,  to  fulfill  both  these 
requirements  elevator  motors  when  of  the  direct-current  type  are 
heavily  over-compounded  to  give  the  series  characteristic  at  starting; 
then,  when  the  motor  is  up  to  speed,  its  series  field  winding  is 
short-circuited  and  it  operates  as  a  shunt  machine.  Recently, 
however,  two-speed  shunt  motors  have  been  employed  for  this 
service,  the  field  being  of  maximum  strength  for  starting,  and 
sparking  prevented  by  use  of  interpoles.  If  only  alternating  cur- 
rent is  available,  the  polyphase  induction  motor  should  be  em- 
ployed, but  for  powerful  starting  torque  slip-ring  control  would  be 
necessary  in  order  to  avoid  very  excessive  currents  and  low  power- 
factors  in  starting  up  or  at  low  speed  (pp.  194  and  202). 

The  fourth  case  (d)  requires  the  motor  to  be  started  and  stopped 
frequently  and  not  rapidly  accelerated,  but  on  the  contrary  slightly 
moved  or  "inched"  forward  at  the  start,  as  in  the  operation  of 
printing  presses,  gun  turrets,  etc.  These  conditions  of  service  are 
satisfied  by  direct-current  compound-wound  motors  provided  with 
double  armature  and  series-parallel  control  (p.  90).  This  character 
of  work  is  also  well  performed  by  having  a  double  or  variable 
potential  source  of  current  supply  for  the  working  motor,  low 
voltage  being  used  for  starting  and  "inching"  and  higher  voltages 


270      ELECTRIC  MOTORS,    THEIR  ACTION   AND   CONTROL. 

for  running.  These  features  are  found  in  the  Bullock  "teazer" 
system,  the  Holmes-Clatworthy  two-motor  method,  or  the  Ward- 
Leonard  motor-dynamo  equipment.  The  last  named,  however, 
being  somewhat  expensive,  is  employed  for  the  operation  of  gun 
turrets,  steel-rolls  and  such  special  service,  in  which  cost  is  a  second- 
ary consideration.  This  "inching"  or  very  slow  operation  can  also 
be  accomplished  by  the  use  of  a  multiple  disk  oil  clutch,  which  can 
be  very  gradually  and  smoothly  applied. 

The  fifth  case  (e)  includes  individual  machine-tool  service,  for 
which  the  maximum  allowable  cutting  or  turning  speed  requires 
the  number  of  revolutions  of  the  work  or  tool  to  vary  inversely  as 
the  diameter  of  the  cut,  maintaining  the  load  at  a  constant  value. 
This  condition  is  satisfied  best  by  direct-current  shunt  motors,  as 
these  are  readily  controlled  in  speed,  as  described  in  Chaps.  V-VII. 

It  is  to  be  noted  that  in  cases  (a)  and  (6),  the  motor  usually 
regulates  automatically  to  maintain  practically  constant  speed.  In 
remaining  cases  (c),  (d)  and  (<?),  on  the  contrary,  the  motor  is  con- 
trolled by  hand  to  give  variable  speeds.  Furthermore,  in  case  (c) 
the  motor  is  under  control  of  the  hand  at  all  times,  while  in  cases 
(d)  and  (e)  the  motor  or  machine  driven  by  it,  after  being  started, 
is  set  to  operate  at  a  desired  speed  for  some  time  and  regulates 
automatically  when  so  adjusted  to  maintain  that  speed. 


For  further  information  on  the  application  of  electric  motors,  the  reader  is  referred 
to  the  following: 

ELECTRIC  DRIVEN  MACHINERY.     Dr.  S.  S.  Wheeler.     Elec.,  N.  Y.,  May,  1898. 

ELECTRIC  POWER  IN  ENGINEERING  WORKS.  Dr.  Louis  Bell.  Eng.  Mag.,  October, 
1899,  January,  1900. 

ELECTRIC  DISTRIBUTION  OF  POWER  IN  WORK  SHOPS.  F.  B.  Crocker.  Fr.  Inst., 
January,  1901. 

THE  CASE  FOR  ELECTRIC  POWER  DISTRIBUTION.  W.  B.  Esson.  Elect.  Eng.,  Lon- 
don, January  n,  1901. 

ELECTRIC  POWER  IN  MFG.  PLANTS.  D.  C.  and  W.  B.  Jackson.  Cassier's  Mag., 
Vol.  26,  1904,  p.  151. 

ELECTRIC  MOTORS  AND  THEIR  APPLICATIONS.  W.  E.  Reed.  Proc.  Eng.  Soc., 
W.  Penn.,  October,  1905. 

INDUSTRIAL  ENGINEERING.    H.  W.  Peck.     Electric  Journal,  Vol.  VI,  1909,  p.  83. 

APPLICATION  OF  MOTORS  TO  MACHINE  TOOLS.  J.  M.  Barr.  Electric  Journal, 
Vol.  II,  1905,  p.  n. 

APPLICATIONS  OF  MOTORS.     Electrical  Record,  June,  1909. 

COST  OF  OPERATING  MACHINE  TOOLS.  A.  G.  Popcke.  Electric  Journal,  Vol.  VI, 
1909,  pp.  674,  757. 

ECONOMIC  FEATURES  OF  ELECTRIC  DRIVE.  Chas.  Robbins.  Trans.  A.  S.  M.  E., 
April,  1910. 


APPENDICES 


APPENDIX  A. 


STANDARDIZATION  RULES 

OF   THE 
AMERICAN  INSTITUTE  OF    ELECTRICAL   ENGINEERS. 


NEW  YORK,  June  17,  1907. 
DR.  SAMUEL  SHELDON, 

PRESIDENT,  AMERICAN  INSTITUTE  ELECTRICAL  ENGINEERS, 

33  West  39th  Street, 

New  York  City. 
DEAR  SIR:  — 

In  accordance  with  a  motion  made  by  Dr.  Steinmetz,  and  duly  carried 
at  the  last  Annual  Convention  of  the  Institute,  at  Milwaukee,  the  Standard- 
ization Rules  have  been  revised  in  form  and  wording  and  in  accordance 
with  various  suggestions  received  from  members  of  the  Institute.  This 
work  has  been  accomplished  by  the  Standards  Committee,  which  has 
held  monthly  meetings  beginning  in  September  last. 

Dr.  Steinmetz'  motion  provided  that  the  Standardization  Rules  when 
completed  by  the  Committee  should  be  submitted  to  the  Board  of  Direc- 
tors for  final  adoption  and  promulgation.  I  therefore  submit  the  revised 
Standardization  Rules  through  you  to  the  Board  of  Directors,  and  re- 
quest that  they  be  formally  approved  and  adopted. 

Respectfully  yours, 

(Signed)  FRANCIS  B.  CROCKER, 

Chairman  Standards  Committee. 

STANDARDS    COMMITTEE. 

FRANCIS  B.  CROCKER,  Chairman,  Columbia  University,  New  York,  N.  Y. 
ARTHUR  W.  BERRESFORD,  Milwaukee.  CHARLES  F.  SCOTT,  Pittsburg,  Pa. 

DUGALD  C.  JACKSON,  Boston,  Mass.  HENRY  G.  STOTT,  New  York,  N.  Y. 

ARTHUR  E.  KENNELLY,  Cambridge,  Mass.       CHARLES  P.  STEINMETZ,  Schenectady. 
C.  O.  MAILLOUX,  New  York,  N.  Y.  SAMUEL  W.  STRATTON,  Washington,  D.  C. 

ROBERT  B.  OWENS,  Montreal,  Can.  ELIHU  THOMSON,  Lynn,  Mass. 

Approved  by  vote  of  the  Board  of  Directors,  June  21,  1907. 

RALPH  W.   POPE, 

Secretary. 

New  York,  June  21,  1907. 

273 


274  APPENDIX. 


PORTIONS  OF  THE 
STANDARDIZATION  RULES   OF  THE  A.   I.   E.  E. 

RELATING  TO  ELECTRIC  MOTORS  AND 
RHEOSTATS. 

The  rules  in  full  may  be  obtained  from  the  Secretary  of  the  Institute. 


L   DEFINITIONS   AND    TECHNICAL   DATA. 

1     Note:  The  following  definitions  and  classifications  are  intended  to 
be  practically  descriptive  and  not  scientifically  rigid. 

E.    MOTORS.     SPEED   CLASSIFICATION. 

45  MOTORS  may,  for  convenience,  be  classified  with  reference  to  their 
speed  characteristic  as  follows: 

46  a.  CONSTANT-SPEED  MOTORS,  in  which  the  speed  is  either  con- 
stant or  does  not  materially  vary;  such  as  synchronous  motors,  induction 
motors  with  small  slip,  and  ordinary  direct-current  shunt  motors. 

47  b.  MULTISPEED  MOTORS  (two-speed,  three-speed,  etc.),  which  can 
be  operated  at  any  cne  of  several  distinct  speeds,  these  speeds  being 
practically  independent  of  the  load,  such  as  motors  with  two  armature 
windings. 

48  c.  ADJUSTABLE-SPEED  MOTORS,  in  which  the  speed  can  be  varied 
gradually  over  a  considerable  range;  but  when  once  adjusted  remains  prac- 
tically unaffected  by  the  load,  such  as  shunt  motors  designed  for  a  con- 
siderable range  of  field  variation. 

49  d.  VARYING-SPEED  MOTORS,  or  motors  in  which  the  speed  varies 
with  the  load,  decreasing  when  the  load  increases;  such  as  series  motors. 

II.    PERFORMANCE    SPECIFICATIONS    AND    TESTS. 
A.    RATING. 

65  RATING  BY  OUTPUT.    All  electrical  apparatus  should  be  rated  by 
output  and  not  by  input.     Generators,  transformers,  etc.,  should  be  rated 
by  electrical  output;  motors  by  mechanical  output. 

66  RATING  IN  KILOWATTS.      Electrical  power  should  be  expressed 
in  kilowatts,  except  when  otherwise  specified. 

67  APPARENT  POWER,  KILOVOLT-AMPERES.    Apparent  power  in  alter- 
nating-current circuits  should  be  expressed  in  kilovolt-amperes  as  dis- 


APPENDIX.  275 

tinguished  from  real  power  in  kilowatts.  When  the  power  factor  is  100 
per  cent,  the  apparent  power  in  kilovolt-amperes  is  equal  to  the  kilowatts. 

68  THE  RATED  (FULL-LOAD)  CURRENT  is  that  current  which,  with 
the  rated  terminal  voltage,  gives  the  rated  kilowatts,  or  the  rated  kilovolt- 
amperes.  In  machines  in  which  the  rated  voltage  differs  from  the  no-load 
voltage,  the  rated  current  should  refer  to  the  former. 

73  NORMAL  CONDITIONS.  The  rating  of  machines  or  apparatus 
should  be  based  upon  certain  normal  conditions  to  be  assumed  as  standard, 
or  to  be  specified.  These  conditions  include  voltage,  current,  power- 
factor,  frequency,  wave  shape  and  speed;  or  such  of  them  as  may  apply 
in  each  particular  case.  Performance  tests  should  be  made  under  these 
standard  conditions  unless  otherwise  specified. 

D.    REGULATION. 

(I)  DEFINITIONS. 

187  DEFINITION.     The  regulation  of  a  machine  or  apparatus  in  regard 
to  some  characteristic  quantity  (such  as  terminal  voltage,  current  or  speed) 
is  the  ratio  of  the  deviation  of  that  quantity  from  its  normal  value  at  rated 
load  to  the  normal  rated  load  value.    The  term  "regulation,"  therefore, 
has  the  same  meaning  as  the  term  "inherent  regulation,'1  occasionally 
used. 

188  CONSTANT  STANDARD.     If  the  characteristic  quantity  is  intended 
to  remain  constant  (e.g.,  constant  voltage,  constant  speed,  etc.)  between 
rated  load  and  no  load,  the  regulation  is  the  ratio  of  the  maximum  varia- 
tion from  the  rated  load  value  to  the  no-load  value. 

189  VARYING  STANDARD.     If  the  characteristic  quantity  is  intended  to 
vary  in  a  definite  manner  between  rated  load  and  no  load,  the  regulation 
is  the  ratio  of  the  maximum  variation  from  the  specified  condition  to  the 
normal  rated-load  value. 

195  In   CONSTANT-SPEED  DIRECT-CURRENT  MOTORS  and  INDUCTION 
MOTORS  the  regulation  is  the  ratio  of  the  maximum  variation  of  speed 
from  its  rated  load  value  (occurring  within  the  range  from  rated  load  to 
no-load)  to  the  rated  load  speed. 

196  The  regulation  of  an  induction  motor  is,  therefore,  not  identical 
with  the  slip  of  the  motor,  which  is  the  ratio  of  the  drop  in  speed  from 
synchronism,  to  the  synchronous  speed. 

206  WAVE  FORM.     In  alternating  apparatus  receiving  electric  power, 
regulation  should  refer  to  a  sine  wave  of  e.m.f.,  except  where  expressly 
specified  otherwise. 

207  EXCITATION.     In  commutating  machines,  rectifying  machines,  and 
synchronous   machines,   such   as   direct-current   generators   and   motors, 


276  APPENDIX. 

alternating-current  and  polyphase  generators,  the  regulation  is  to  be  deter- 
mined under  the  following  conditions: 

(1)  At  constant  excitation  in  separately  excited  fields. 

(2)  With  constant  resistance  in  shunt-field  circuits,  and 

(3)  With  constant  resistance  shunting  series-field  circuits;  i.e.,  the  field 
adjustment  should  remain  constant,  and  should  be  so  chosen  as  to  give 
the  required  full-load  voltage  at  full-load  current. 


E.    INSULATION. 

(I)  INSULATION  RESISTANCE. 

210  INSULATION  RESISTANCE  is  the  ohmic  resistance  offered  by  an 
insulating  coating,  cover,  material  or  support  to  an  impressed  voltage, 
tending  to  produce  a  leakage  of  current  through  the  same. 

211  OHMIC  RESISTANCE  AND  DIELECTRIC  STRENGTH.     The  ohmic 
resistance  of  the  insulation  is  of  secondary  importance  only,  as  compared 
with  the  dielectric  strength,  or  resistance  to  rupture  by  high  voltage. 
Since  the  ohmic  resistance  of  the  insulation  can  be  very  greatly  increased 
by  baking,  but  the  dielectric  strength  is  liable  to  be  weakened  thereby,  it 
is  preferable  to  specify  a  high  dielectric  strength  rather  than  a  high  insula- 
tion resistance.    The  high-voltage  test  for  dielectric  strength  should  always 
be  applied. 

212  RECOMMENDED  VALUE  OF  RESISTANCE.    The  insulation  resist- 
ance of  complete  apparatus  should  be  such  that  the  rated  voltage  of  the 
apparatus  will  not  send  more  than  y^oio OF  of  tne  rated-load  current,  at 
the  rated  terminal  voltage,  through  the  insulation.     Where  the  value  found 
in  this  way  exceeds  i  megohm,  it  is  usually  sufficient. 

213  INSULATION  RESISTANCE  TESTS  should,  if  possible,  be  made  at 
the  pressure  for  which  the  apparatus  is  designed. 

(II)  DIELECTRIC  STRENGTH. 

(A)  TEST  VOLTAGES. 

214  DEFINITION.    The  dielectric  strength  of  an  insulating  wall,  coat- 
ing, cover  or  path  is  measured  by  the  voltage  which  must  be  applied  to  it 
in  order  to  effect  a  disruptive  discharge  through  the  same. 

215  BASIS  FOR  DETERMINING   TEST   VOLTAGES.      The  test  voltage 
which  should  be  applied  to  determine  the  suitability  of  insulation  for  com- 
mercial operation  is  dependent  upon  the  kind  and  size  of  the  apparatus 
and  its  normal  operating  voltage,  upon  the  nature  of  the  service  in  which 
it  is  to  be  used,  and  the  severity  of  the  mechanical  and  electrical  stresses 
to  which  it  may  be  subjected.    The  voltages  and  other  conditions  of  test 
which  are  recommended  have  been  determined  as  reasonable  and  proper 


APPENDIX.  277 

for  the  great  majority  of  cases  and  are  proposed  for  general  adoption, 
except  when  specific  reasons  make  a  modification  desirable. 

216  CONDITION  OF  APPARATUS  TO  BE  TESTED.     Commercial  tests 
should,  in  general,  be  made  with  the  completely  assembled  apparatus  and 
not  with  individual  parts.    The  apparatus  should  be  in  good  condition, 
and  high-voltage  tests,  unless  otherwise  specified,  should  be  applied  be- 
fore the  machine  is  put  into  commercial  service,  and  should  not  be  applied 
when  the  insulation  resistance  is  low  owing  to  dirt  or  moisture.     High- 
voltage  tests  should,  in  general,  be  made  at  the  temperature  assumed 
under  normal  operation.     High-voltage  tests  considerably  in  excess  of  the 
normal   voltages  to  determine  whether  specifications    are   fulfilled    are 
admissible  on  new  machines  only. 

217  POINTS  OF  APPLICATION  OF  VOLTAGE.    The  test  voltage  should 
be  successively  applied  between  each  electric  circuit  and  all  other  electric 
circuits  including  conducting  material  in  the  apparatus. 

218  The   FREQUENCY  of   the  alternating-current  test  voltage  is,  in 
general,  immaterial  within  commercial  ranges.     When,  however,  the  fre- 
quency has  an  appreciable  effect,  as  in  alternating-current  apparatus  of 
high  voltage  and  considerable  capacity,  the  rated  frequency  of  the  appara- 
tus should  be  used. 

219  TABLE    OF    TESTING    VOLTAGES.    The    following   voltages    are 
recommended  for  testing  all  apparatus,  lines  and  cables,  by  a  continued 
application  for  one  minute.    The  test  should  be  with  alternating  voltage 
having  an  effective  value  (or  root  mean  square  referred  to  a  sine  wave  of 
voltage)  given  in  the  table  and  preferably  for  tests  of  alternating  apparatus 
at  the  normal  frequency  of  the  apparatus. 

Rated  Terminal  Voltage  of  Circuit.  Rated  Output.     Testing  Voltage. 

220  Not  exceeding  400  volts Under  10  kw 1,000  volts. 

10  kw.  and  over 1,500  " 

400  and  over,  but  less  than  800  volts.  . .  .  Under  10  kw 1,500  " 

"      "  "  ""....  10  kw.  and  over 2,000  " 

800"       "  "      1,200     "    ....Any 3,500  " 

1,200"      "  "       2,500     "    Any 5,000  " 

2,500   "       "  Any. .  .Double  the  normal  rated 

voltages. 

222  EXCEPTION.  —  FIELD  WINDINGS.  The  tests  for  field  windings 
should  be  based  on  the  rated  voltage  of  the  exciter  and  the  rated  output  of 
the  machine  of  which  the  coils  are  a  part.  Field  windings  of  synchronous 
motors  and  converters,  which  are  to  be  started  by  applying  alternating 
current  to  the  armature  when  the  field  is  not  excited  and  a  high  voltage  is 
induced  in  the  field  windings,  should  be  tested  at  5000  volts. 


278  APPENDIX. 


F.     CONDUCTIVITY. 

260  COPPER.  The  conductivity  of  copper  in  electric  wires  and  cables 
should  not  be  less  than  98%  of  Matthiessen's  standard  of  conductivity,  as 
defined  in  the  Copper  Wire  Table  of  the  AMERICAN  INSTITUTE  OF  ELEC- 
TRICAL ENGINEERS. 


G.    RISE   OF  TEMPERATURE. 

(I)  MEASUREMENT  OF  TEMPERATURE. 
(A)  METHODS. 

261  There  are  two  methods  in  common  use  for  determining  the  rise 
in  temperature,  viz.:  (i)  by  thermometer,  and  (2)  by  increase  in  resistance 
of  an  electric  circuit. 

262  i.  By    THERMOMETER.     The    following   precautions    should    be 
observed  in  the  use  of  thermometers: 

263  a.  PROTECTION.     The   thermometers   indicating   the  room   tem- 
perature should  be  protected  from  thermal  radiation  emitted  by  heated 
bodies,  or  from  draughts  of  air  or  from  temporary  fluctuations  of  tem- 
perature.    Several  room  thermometers  should  be  used.     In  using  the 
thermometer  by  applying  it  to  a  heated  part,  care  should  be  taken  so  to 
protect  its  bulb  as  to  prevent  radiation  from  it,  and,  at  the  same  time,  not 
to  interfere  seriously  with  the  normal  radiation  from  the  part  to  which  it 
is  applied. 

264  b.  BULB.     When  a  thermometer  is  applied  to  the  free  surface  of 
a  machine,  it  is  desirable  that  the  bulb  of  the  thermometer  should  be 
covered  by  a  pad  of  definite  area.     A  convenient  pad  may  be  formed  of 
cotton  waste  in  a  shallow  circular  box  about  one  and  a  half  inches  in 
diameter,  through  a  slot  in  the  side  in  which  the  thermometer  bulb  is 
inserted.     An  unduly  large  pad  over  the  thermometer  tends  to  interfere 
with  the  natural  liberation  of  heat  from  the  surface  to  which  the  ther- 
mometer is  applied. 

265  2.  By  INCREASE  IN  RESISTANCE.    The  resistance  may  be  meas- 
ured either  by  Wheatstone  bridge,  or  by  drop-of-potential  method.     A 
temperature  coefficient  of  0.42  per  cent  per  degree  C.,  from  and  at  o°  C., 
may  be  assumed  for  copper. 

The  temperature-coefficients  from  and  at  each  degree  cent,  between 
o°  C.  and  50°  C.  are  given  in  Appendix  E.  The  temperature  rise  may  be 
determined  either  (i)  by  dividing  the  percentage  increase  of  initial  re- 
sistance by  the  temperature-coefficient  for  the  initial  temperature  ex- 
pressed in  per  cent;  or  (2)  by  multiplying  the  increase  5n  per  cent  of  the 


APPENDIX.  279 

initial  resistance  by  238.1  plus  the  initial  temperature  in  degrees  C.,  and 
then  dividing  the  product  by  100. 

266  3.   COMPARISON  OF  METHODS.     In  electrical  conductors,  the  rise 
of  temperature  should  be  determined  by  their  increase  of  resistance  where 
practicable.     Temperature  elevations  measured  in  this  way  are  usually  in 
excess  of  temperature  elevations  measured  by  thermometers.     In  very  low 
resistance  circuits,  thermometer  measurements  are  frequently  more  reli- 
able than  measurements  by  the  resistance  method.     Where  a  thermometer 
applied  to  a  coil  or  winding   indicates  a  higher  temperature   elevation 
than  that  shown  by  resistance  measurement,  the  thermometer  indication 
should  be  accepted. 

(B)  NORMAL  CONDITIONS  FOR  TESTS. 

267  i.   DURATION  OF  TESTS.     The  temperature  should  be  measured 
after  a  run  of  sufficient  duration  for  the  apparatus  to  reach  a  practically 
constant  temperature.     This  is  usually  from  6  to  18  hours,  according  to 
the  size  and  construction  of  the  apparatus.     It  is  permissible,  however,  to 
shorten  the  time  of  the  test  by  running  a  lesser  time  on  an  overload  in 
current  and  voltage,  then  reducing  the  load  to  normal,  and  maintaining 
it  thus  until  the  temperature  has  become  constant. 

268  2.   ROOM  TEMPERATURE.    The  rise  of  temperature   should  be 
referred  to  the  standard  condition  of  a  room  temperature  of  25°  C. 

269  TEMPERATURE   CORRECTION.     If  the  room   temperature  during 
the  test  differs  from  25°  C.,  correction  on  account  of  difference  in  resist- 
ance should  be  made  by  changing  the  observed  rise  of  temperature  by  one- 
half  per  cent,  for  each  degree  C.     Thus  with  a  room  temperature  of  35°  C., 
the  observed  rise  of  temperature  has  to  be  decreased  by  5  per  cent,  and 
with  a  room  temperature  of  15°  C.,  the  observed  rise  of  temperature  has 
to  be  increased  by  5  per  cent.     In  certain  cases,  such  as  shunt-field  cir- 
cuits without  rheostat,  the  current  strength  will  be  changed  by  a  change  of 
room  temperature.     The  heat  production  and  dissipation  may  be  thereby 
affected.     Correction  for  this  should  be  made  by  changing  the  observed 
rise  in  temperature  in  proportion  as  the  PR  loss  in  the  resistance  of  the 
apparatus  is  altered  owing  to  the  difference  in  room  temperature. 

270  3.   BAROMETRIC  PRESSURE.    VENTILATION.     A  barometric  pres- 
sure of  760  mm.  and  normal  conditions  of  ventilation  should  be  considered 
as  standard,  and  the  apparatus  under  test  should  neither  be  exposed  to 
draught  nor  enclosed,  except  where  expressly  specified.     The  barometric 
pressure  needs  to  be  considered  only  when  differing  greatly  from  760  mm. 

271  BAROMETRIC    PRESSURE    CORRECTION.      When    the    barometric 
pressure  differs  greatly  from  the  standard  pressure  of  760  mm.  of  mer- 
cury, as  at  high  altitudes,  a  correction  should  be  applied.     In  the  absence 
of  more  accurate  data,  a  correction  of  i%  of  the  observed  rise  in  tempera- 


280  APPENDIX. 

ture  for  each  10  mm.  deviation  from  the  760  mm.  standard  is  recom- 
mended.    For  example,  at  a  barometric  pressure  of  680  mm.  the  observed 

760  —  680 

rise  of  temperature  is  to  be  reduced  by =  8  %. 

10 

(II)  LIMITING   TEMPERATURE  RISE. 

272  GENERAL.     The  temperature  of  electrical  machinery  under  reg- 
ular service  conditions  should  never  be  allowed  to  remain  at  a  point  at 
which  permanent  deterioration  of  its  insulating  material  takes  place. 

273  LIMITS  RECOMMENDED.     It  is  recommended  that  the  following 
maximum  values  of  temperature  elevation,  referred  to  a  standard  room 
temperature  of  25  degrees  centigrade,  at  rated  load  under  normal  con- 
ditions of  ventilation  or  cooling,  should  not  be  exceeded. 

(A)  MACHINES  IN  GENERAL. 

274  In  commuting  machines,  rectifying  machines,  pulsating-current 
generators,   synchronous  machines,   synchronous  commutating  machines 
and  unipolar  machines,  the  temperature  rise  in  the  parts  specified  should 
not  exceed  the  following: 

275  Field  and  armature,  50°  C. 

276  Commutator  and  brushes,  by  thermometer,  55°  C. 

277  Collector  rings,  65°  C.   x. 

278  Bearings  and  other  parts  of  machine,  by  thermometer,  40°  C. 

279  (B)  ROTARY    INDUCTION    APPARATUS.     The    temperature    rise 
should  not  exceed  the  following: 

280  Electric  circuits,  50°  C.,  by  resistance. 

281  Bearings  and  other  parts  of  the  machine  40°  C.,  by  thermometer. 

282  In   squirrel-cage  or  short-circuited  armatures,   55°  C.,  by  ther- 
mometer, may  be  allowed. 

(D)  RHEOSTATS. 

287  In  RHEOSTATS,  HEATERS  and  other  electrothermal  apparatus,  no 
combustible  or  inflammable  part  or  material,  or  portion  liable  to  come  in 
contact  with  such  material,  should  rise  more  than  50°  C.  above  the  sur- 
rounding air  under  the  service  conditions  for  which  it  is  designed. 

288  a.  PARTS  OF  RHEOSTATS.    Parts  of  rheostats  and  similar  appa- 
ratus rising  in  temperature,  under  the  specified  service  conditions,  more 
than  50°  C.  should  not  contain  any  combustible  material,  and  should  be 
arranged  or  installed  in  such  a  manner  that  neither  they,  nor  the  hot  air 
issuing  from  them,  can  come  in  contact  with  combustible  material. 

(E)  LIMITS  RECOMMENDED  IN  SPECIAL  CASES. 

289  a.  HEAT  RESISTING  INSULATION.     With  apparatus  in  which  the 
insulating  materials  have  special  heat-resisting  qualities,  a  higher  temper- 
ature elevation  is  permissible. 


APPENDIX.  281 

290  b.  HIGH  AIR  TEMPERATURE.     In  apparatus  intended  for  service 
in  places  of  abnormally  high  temperature,  a  lower  temperature  elevation 
should  be  specified. 

291  c.  APPARATUS  SUBJECT  TO  OVERLOAD.    In  apparatus  which  by 
the  nature  of  its  service  may  be  exposed  to  overload,  or  is  to  be  used  in 
very  high  voltage  circuits,  a  smaller  rise  of  temperature  is  desirable  than 
in  apparatus  not  liable  to  overloads  or  in  low-voltage  apparatus.     In 
apparatus  built  for  conditions  of  limited  space,  as  railway  motors,  a  higher 
rise  of  temperature  must  be  allowed. 

292  d.  APPARATUS    FOR    INTERMITTENT    SERVICE.    In    the   case   of 
apparatus  intended  for  intermittent  service,  except  railway  motors,  the 
temperature  elevation  which  is  attained  at  the  end  of  the  period  corre- 
sponding to  the  term  of  rated  load  should  not  exceed  the  values  specified 
for  machines  in  general.     In  such  apparatus  the  temperature  elevation, 
including  railway  motors,  should  be  measured  after  operation,  under  as 
nearly  as  possible  the  conditions  of  service  for  which  the  apparatus  is 
intended,  and  the  conditions  of  the  test  should  be  specified. 


H.     OVERLOAD   CAPACITIES. 

293  PERFORMANCE  WITH  OVERLOAD.    All  apparatus  should  be  able 
to  carry  the  overload  hereinafter  specified  without  serious  injury  by  heat- 
ing, sparking,  mechanical  weakness,  etc.,  and  with  an  additional  tem- 
perature rise  not  exceeding  15°  C.,  above  those  specified  for  rated  loads, 
the  overload  being  applied  after  the  apparatus  has  acquired  the  tem- 
perature corresponding  to  rated  load  continuous  operation.     Rheostats 
to  which  no  temperature  rise  limits  are  attached  are  naturally  exempt 
from  this  additional  temperature  rise  of  15°  C.  under  overload  specified 
in  these  rules. 

294  NORMAL  CONDITIONS.     Overload  guarantees  should  refer  to  nor- 
mal conditions  of  operation  regarding  speed,  frequency,  voltage,  etc.,  and 
to  non-inductive  conditions  in  alternating  apparatus,  except  where  a  phase 
displacement  is  inherent  in  the  apparatus. 

295  OVERLOAD  CAPACITIES  RECOMMENDED.    The  following  overload 
capacities  are  recommended: 

296  a.   GENERATORS.     Direct-current  generators  and  alternating-cur- 
rent generators,  25  per  cent  for  two  hours. 

297  b.   MOTORS.     Direct-current  motors,   induction  motors  and  syn- 
chronous motors,  not  including  railway  and  other  motors  intended  for 
intermittent  service,  25  per  cent  for  two  hours,  and  50  per  cent  for  one 
minute. 


282  APPENDIX. 

301  /.  A  CONTINUOUS-SERVICE  RHEOSTAT,  such  as  an  armature-  or 
field-regulating  rheostat,  should  be  capable  of  carrying  without  injury  for 
two  hours,  a  current  25  per  cent  greater  than  that  at  which  it  is  rated.     It 
should  also  be  capable  of  carrying  for  one  minute  a  current  50  per  cent 
greater  than  its  rated  load  current,  without  injury.     This  excess  of  capacity 
is  intended  for  testing  purposes  only,  and  this  margin  of  capacity  should 
not  be  relied  upon  in  the  selection  of  the  rheostat. 

302  g.  An  INTERMITTENT  SERVICE  OR  MOTOR-STARTING  RHEOSTAT  is 
used  for  starting  a  motor  from  rest  and  accelerating  it  to  rated  speed. 
Under  ordinary  conditions  of  service,  and  unless  expressly  stated  otherwise, 
a  motor  is  assumed  to  start  in  fifteen  seconds  and  with  150%  of  rated 
current  strength.     A  motor-starter  should  be  capable  of  starting  the  motor 
under  these  conditions  once  every  four  minutes  for  one  hour. 

303  (a)  This  TEST  may  be  carried  out  either  by  starting  the  motor 
at  four-minute  intervals,  or  by  placing  the  starter  at  the  normal  temper- 
ature across  the  maximum  voltage  for  which  it  is  marked,  and  moving  the 
lever  uniformly  and  gradually  from  the  first  to  the  last  position  during  a 
period  of  fifteen  seconds,  the  current  being  maintained  substantially  con- 
stant at  said  50%  excess  by  introducing  resistance  in  series  or  by  other 
suitable  means. 

304  (6)  OTHER  RHEOSTATS  FOR  INTERMITTENT-SERVICE  are  employed 
under  such  special  and  varied  conditions  that  no  general  rules  are  appli- 
cable to  them. 

IV.     GENERAL   RECOMMENDATIONS. 

313  NAME  PLATES.     All  electrical  apparatus  should  be  provided  with 
a  name  plate  giving  the  manufacturer's  name,  the  voltage  and  the  current 
in  amperes  for  which  it  is  designed.     Where  practicable,  the  kilowatt 
capacity,  character  of  current,  speed,  frequency,  type,  designation  and 
serial  number  should  be  added. 

314  DIAGRAMS    OF    CONNECTIONS.    All    electrical    apparatus    when 
leaving  the  factory  should  be  accompanied  by  a  diagram  showing  the 
electrical  connections  and  the  relation  of  the  different  parts  in  sufficient 
detail  to  give  the  necessary  information  for  proper  installation. 

315  RHEOSTAT  DATA.     Every  rheostat  should  be  clearly  and  perma- 
nently marked  with  the  voltage  and  amperes,  or  ranges,  for  which  it  is 
designed. 

(I)  RATING.    RAILWAY    MOTORS. 

325  INTRODUCTORY  NOTE  ON  RATING.  Railway  motors  usually 
operate  in  a  service  in  which  both  the  speed  and  the  torque  developed  by 
the  motor  are  varying  almost  continually.  The  average  requirements, 


APPENDIX.  283 

however,  during  successive  hours  in  a  given  class  of  service  are  fairly 
uniform.  On  account  of  the  wide  variation  of  the  instantaneous  loads,  it  is 
impracticable  to  assign  any  simple  and  definite  rating  to  a  motor  which 
will  indicate  accurately  the  absolute  capacity  of  a  given  motor  or  the 
relative  capacity  of  different  motors  under  service  conditions.  It  is  also 
impracticable  to  select  a  motor  for  a  particular  service  without  much  fuller 
data  with  regard  both  to  the  motor  and  to  the  service  than  is  required,  for 
example,  in  the  case  of  stationary  motors  which  run  at  constant  speeds. 

326  SCOPE  OF  NOMINAL  RATING.     It  is  common  usage  to  give  rail- 
way motors  a  nominal  rating  in  horsepower  on  the  basis  of  a  one-hour 
test.     As  above  explained,  a  simple  rating  of  this  kind  is  not  a  proper 
measure  of  service  capacity.     This  nominal  rating,   however,   indicates 
approximately  the  maximum  output  which  the  motor  should  ordinarily 
be  called  upon  to  develop  during  acceleration.     Methods  of  determining 
the  continuous  capacity  of  a  railway  motor  for  service  requirements  are 
given  under  a  subsequent  heading. 

327  The  NOMINAL  RATING  of  a  railway  motor  is  the  horsepower  out- 
put at  the  car-axle,  that  is,  including  gear  and  other  transmission  losses, 
which  gives  a  rise  of  temperature  above  the  surrounding  air  (referred  to  a 
room  temperature  of  25  degrees  cent.)  not  exceeding  90  degrees  cent,  at 
the  commutator  and  75  degrees  cent,  at  any  other  part  after  one  hour's 
continuous  run  at  its  rated  voltage  (and  frequency,  in  the  case  of  an  alter- 
nating-current motor)  on  a  stand,  with  the  motor-covers  removed,  and 
with  natural  ventilation.     The  rise  in  temperature  is  to  be  determined 
by  thermometer,  but  the  resistance  of  no  electrical  circuit  in  the  motor 
shall  increase  more  than  40%  during  the  test. 

(II)  SELECTION  OF  MOTOR  FOR  SPECIFIED  SERVICE. 

328  GENERAL  REQUIREMENTS.     The  suitability  of  a  railway  motor 
for  a  specified  service  depends  upon  the  following  considerations: 

329  a.   Mechanical  ability  to  develop  the  requisite  torque  and  speeds 
as  given  by  its  speed-torque  curve. 

330  b.  Ability  to  commutate  successfully  the  current  demanded. 

331  c.  Ability  to  operate  in  service  without  occasioning  a  temperature 
rise  in  any  part  which  will  endanger  the  life  of  the  insulation. 

332  OPERATING   CONDITIONS,   TYPICAL   RUN.    The   operating   con- 
ditions which  are  important  in  the  selection  of  a  motor  include  the  weight 
of  load,  the  schedule  speed,  the  distance  between  stops,  the  duration  of 
stops,  the  rate  of  acceleration  and  of  braking  retardation,  the  grades  and 
the  curves.    With  these  data  at  hand,  the  outputs  which  are  required  of 
the  motor  may  be  determined,  provided  the  service  requirements  are 


284  APPENDIX. 

within  the  limits  of  the  speed-torque  curve  of  the  motor.  These  outputs 
may  be  expressed  in  the  form  of  curves  giving  the  instantaneous  values 
of  current  and  of  voltage  which  must  be  applied  to  the  motor.  Such 
curves  may  be  laid  out  for  the  entire  line,  but  they  are  usually  constructed 
only  for  a  certain  average  or  typical  run,  which  is  fairly  representative  of 
the  conditions  of  service.  To  determine  whether  the  motor  has  sufficient 
capacity  to  perform  the  service  safely,  further  tests  or  investigations  must 
be  made. 

333  CAPACITY  TEST  or  RAILWAY  MOTOR  IN  SERVICE.     The  capacity 
of  a  railway  motor  to  deliver  the  necessary  output  may  be  determined  by 
measurement  of  its  temperature  after  it  has  reached  a  maximum  in  serv- 
ice.    If  a  running  test  cannot  be  made  under  the  actual  conditions  of 
service,  an  equivalent  test  may  be  made  in  a  typical  run  back  and  forth, 
under  such  conditions  of  schedule  speed,  length  of  run,  rate  of  acceleration, 
etc.,  that  the  test  cycle  of  motor  losses  and  conditions  of  ventilation  are 
essentially  the  same  as  would  be  obtained  in  the  specified  service. 

334  METHODS    OF    COMPARING    MOTOR    CAPACITY    WITH    SERVICE 
REQUIREMENTS.     Where  it  is  not  convenient  to  test  motors  under  actual 
service  conditions  or  in  an  equivalent  typical  run,  recourse  may  be  had 
to  one  of  the  two  following  methods  of  determining  temperature  rise  now 
in  general  use: 

335-  i.  METHOD  BY  LOSSES  AND  THERMAL  CAPACITY  CURVES.  The 
heat  developed  in  a  railway  motor  is  carried  partly  by  conduction  through 
the  several  parts  and  partly  by  convection  through  the  air  to  the  motor- 
frame  whence  it  is  distributed  to  the  outside  air.  As  the  temperature  of 
the  several  parts  is  thus  dependent  not  only  upon  their  own  internal  losses 
but  also  upon  the  temperature  of  neighboring  parts,  it  becomes  necessary 
to  determine  accurately  the  actual  value  and  distribution  of  losses  in  a 
railway  motor  for  a  given  service  and  reproduce  them  in  an  equivalent 
test-run.  The  results  of  a  series  of  typical  runs  expressed  in  the  form  of 
thermal  capacity  curves  will  give  the  relation  between  degrees  rise  per 
watt  loss  in  the  armature  and  in  the  field  for  all  ratios  of  losses  between 
them  met  with  in  the  commercial  application  of  a  given  motor. 

336  This  method  consists,  therefore,  in  calculating  the  several  internal 
motor  losses  in  a  specified  service  and  determining  the  temperature  rise 
with  these  losses  from  thermal  capacity  curves  giving  the  degrees  rise 
per  watt  loss  as  obtained  in  experimental  track  tests  made  under  the  same 
conditions  of  ventilation. 

337  The  following  motor  losses  cause  its  heating  and  should  be  care- 
fully determined -for  a  given  service:  PR  in  the  field;  PR  in  the  armature; 
PR  in  the  brush  contracts,  core  loss  and  brush  friction. 

338  The  loss  in  the  bearings  (in  the  case  of  geared  motors)  also  adds 


APPENDIX.  285 

somewhat  to  the  motor-heating,  but  owing  to  the  variable  nature  of  such 
losses  they  are  generally  neglected  in  making  calculations. 

339  2.   METHOD  BY  CONTINUOUS  CAPACITY  OF  MOTORS.     The  essen- 
tial losses  in  the  motor,  as  found  in  the  typical  run,  are  in  most  cases  those 
in  the  motor  windings  and  in  the  core.     The  mean  service  conditions  may 
be  expressed  in  terms  of  the  current  which  would  produce  the  same  losses 
in  the  motor  windings  and  the  voltage  which,  with  that  current,  would 
produce  the  same  core  losses  as  the  average  in  service.     The  continuous 
capacity  of  the  motor  is  given  in  terms  of  the  amperes  which  it  will  carry 
when  run  on  a  testing  stand  —  with  covers  on  or  off,  as  specified  —  at 
different  voltages,  say,  40,  60,  80  and  100  per  cent  of  the  rated  voltage  — 
with  a   temperature  rise  not  exceeding  90  degrees  at   the  commutator 
and  75  degrees  at  any  other  part,  provided  the  resistance  of  no  electric 
circuit  in  the  motor  increases  more  than  40  per  cent.     A  comparison  of 
the  equivalent  service  conditions  with  the  continuous  capacity  of  the  motor 
will  determine  whether  the  service  requirements  are  within  the  safe  capacity 
of  the  motor. 

340  This  method  affords  a  ready  means  of  determining  whether  a 
specified  service  is  within  the  capacity  of  a  given  motor  and  it  is  also  a 
convenient  approximate  method  for  comparing  the  service  capacities  of 
different  motors. 


INDEX 


Page 
A 

Adams,  Prof.  C.  A 176 

Adams,  W.  G 125 

Adjustable-speed  Motors 267,  273 

Advantages  of  Electric  Power .    .    4 

Alternating-current  Motors 

History  of 125 

Polyphase  Induction 126,  165 

Repulsion      130,  257 

Series 129,  240 

Shunt 239 

Single -phase  Induction 214 

Synchronous 125,  130 

Alternators 

Parallel  Connection 125,  136 

Series  Connection 133 

Anthony,  Prof.  W.  A 130 

Application  of  Motors 266 

Arago.F.J 126 

Armature 

Heating 13,  26 

Resistance 18 

Resistance  Control 38 

Armature  Reaction 21 

Balancing  Winding 50,  58 

Compensation  of      48,  58 

Effect  on  Speed 25 

Auto- transformer „.    .   195 

Auxiliary  Poles 49,  60 

B 

Bailey,  W 126 

Balancer  Sets,  Multiple  Voltage 82 

Balancing  Actions  of  Synchronous  Motors     .   161 

Balancing  Windings,  Ryan,  H.  J 50,  58 

Ball  Bearings 65 

Barlow's  Wheel 1 

Boost  and  Retard  System 83 

Boucherat,  P 197 

Bradley,  C.  S 124,  127 

Brush-contact  Resistance 16,  19 

Brush  Drop 18 

Brush  Shifting      45 

Bullock  Elec.  Mfg.  Co. 

Adjustable-speed  Shunt  Motors  ...  60,  92 

Multiple-voltage  System 78 

Teaser  System     85 


Cascade  Control 207 

Chain  Drive     .  .  268 


Page 

Characteristic  Curves  of 

Compound  Motors 120 

Induction  Motors    .....      181,189,228 

Series  Motors 106,  252 

Shunt  Motors 50,  74 

Synchronous  Motors 149, 154, 159 

Circle  Diagram  of 

A.  C.  Series  Motor 250 

Heyland,  A 182 

McAllister,  A.  S 186,  230 

Polyphase-Induction  Motors      .    .      182,  186 

Repulsion  Motor      261 

Single-phase  Induction  Motor 230 

Synchronous  Motor 152 

Classification  of  Motors      4,  123,  267 

Commercial  Electric  Co. 

Double  Armature  Control 90,  92 

Commutating  Fringe 49 

Commutation  Lug 48 

Commutation  Poles 49,  60 

Compensation 

Armature  Reaction 49 

Thompson-Ryan      49 

Windings 49,  254,  259,  262 

Compensator  Starter 194 

Compound-wound  Motors 118 

Condenser-Compensator 235 

Condict,  G.  H Ill 

Constant 

Current  Motor 114 

H.  P.  Motors 47,  74 

Torque  Motors 47,  74 

Controller 

Drum  and  Master 113 

G.  E.  TypeK 110 

Resistance     38,  109 

Counter  E.M.F 14,  16,  46 

Crocker-Wheeler  Co. 

Multiple- Voltage  System 77 

Current  Equivalent 

Single  Phase 186 

D 

Damping  Grids 162 

Data  of 

Induction  Motors 192,230 

Shunt  Motors 20 

Deprez,  M 126 

Design  Features  of  Series  A.  C.  Motors  .    .    .  244 

Design  Features  Shunt  Motors 47 

Design  of  Resistance  Controller 39 


287 


288 


INDEX 


Diehl  Co.  Field  Control 

Dielectric  Strength . 

Differentially  Wound  Motors    .    .    . 

Direct  Current  Motors 

Double-armature  Control  Methods  . 
Double-frequency  Currents  .... 
Drum  and  Master  Controller  .  .  . 
Dunn,  G.  S 


Page 

.    .      68 

.    .  275 

118, 121 

4 


222 

113 

66 


Edison,  Reluctance  Control 67 

Efficiency 

Calculation  of       22 

Field-controlled  Motors  .     52,  55,  59,  65,  73 

Induction  Motors 193,  231 

Multiple-voltage  Control 81 

Name  Plate 22 

Resistance  Control 41 

Series  Motor  Control 103,  113 

Eichmeyer,  R 130 

Electric  Drive 

Advantages  of 5 

Applications  of 266 


Reliability  of 
Speed  Control 


Electro-Dynamic  Co 61 

Equivalent  Single-phase 


Current 


186 


Resistance     .    . 186 

Equivalent  Transformer 183 


Faraday's  Disk 1 

Ferraris,  Prof.  G 127,  165,  225 

Field 

Heating  of  Windings 12,  26 

Reluctance  Variation 67 

Rotating 126,  165,  216 

Speed  Control 46,  111 

Flux  Distribution 51,  53,  57,  64 


Gears 105,  366 

General  Electric  Co. 

Double-armature  Control 90 

Induction  Motor      232 

Series  Motor  Type  69 105 

Type  K  Controller 110 


Heating 

Armature 13,  26 

Field       12,  26 

Limits 12,  106,  109,  277,  279 

Heyland,  A    .    .    . 183,  236 

Heyland  Circle  Diagram 183 

Heyland  Induction  Motors 235 

History  of  Motors 124 

Holmes-Clatworthy  System 89 

Hopkinson,  J 124 

Horse-power  Current  Curve 102 

Hunting  of  Motors     161 


Induction  Motor,  Polyphase      . 

Advantages  of  Three-phase 

Cascade  Connection  .     .     . 

Characteristic  Curves     .    . 

Circle  Diagram 

Compensator  for     .... 

Construction  of 

Development  of 

Efficiency  of 

Equivalent  Single-phase     . 

Equivalent  Transformer     . 

Field-Magnetic 

Flux  Equation 

Leakage  Component  .    .    . 

Current 

Reactance    

Locked  Current 

Saturation  Curve    .    . 

Magnetizing  Current      .    . 
Factor      

Poles 

Power  Factor 

Pull-out  Torque 

Resistance  Control  .... 

Rotors  for 

Slip 

Slip-ring  Control 

Speed  Control 

Starting  of 

Starting  Torque 

Stator  for 

Torque  and  Resistance      .... 

Torque- Voltage 

Induction  Motor,  Single-phase      .    .    . 

Absence  of  Starting  Torque  .    .    . 

Brown,  Boveri  &  Co.  Types  .    .    . 

Characteristic  Curves 

Condenser  Compensator     .    .    .    . 

Form  of  Field 

G.  E.  Starter 

Methods  of  Starting 

Rotating  Field 

Rotor  Currents 

Slip 

Slip-rotor  Resistance      

Split-phase 

Starting  of 

Steinmetz,  C.  P 

Tesla  Patent 

Wagner  Electric  Co 

Insulation  Resistance      

Interpoles      

Interpole  Motor 

Iron  Loss,  Series  Motor     


177, 


165, 


Page 

127,  165 
.  .  169 
.  .  207 
.  .  189 
182,  186 
.  .  190 
.  .  165 
.  .  127 
.  .  192 
.  .  185 
.  .  186 
.  .  165 
.  .  174 
.  .  176 
.  .  178 
.  .  176 
.  .  187 
.  .  187 
.  .  175 
.  .  175 
.  .  173 
178,  192 
181,  192 
173,  207 
173,  197 
169,  192 
.  .  173 
.  .  204 
.  .  194 

181,  202 
165,  169 
180, 201 

182,  212 
.  .  214 
.  .  214 
.  .  233 
.  .  230 
.  .  235 
.  .  219 
.  .  234 
.  .  232 
.  .  216 
.  .  222 
.  .  231 
.  .  227 
.  .  232 
.  .  232 

232,  234 
.  .  232 
.  .  232 
.  .  275 
.  .  49 

,  60,  111 
.  242 


Jackson,  Prof.  D.  C 130 

Jacobi  Motor .       1 

L 

Lamme,  G.  B 130,241 

Leakage 

Current 178 


INDEX 


289 


Page 
Leakage  —  Continued. 

Flux 176 

Primary 176 

Reactance 170,  176 

Reduction  of  Power  Factor  by      ....   178 

Secondary 176 

Lincoln  Motor      72 

Load  Factor 5 

Locked  Current 187 

Locked  Saturation  Curve 187 

Losses,  Division  of 24 

M 

McAllister,  A.  S 185,230 

Magneto  Electric  Co 52 

Master  Control 113 

Motors 

Advantages  of 5 

Application  of 266 

Classes  of  Service 266 

Compensated 48,  55,  254,  262 

Definition 1 

Compound 118 

Induction 127,  165,  214 

Interpole 49,  60,  111 

Series 94,  241 

Shunt 10,  239 

Speed  Classification 273 

Synchronous 125,  131 

Rating  of 273 

Repulsion 130,  257 

Types  of 4,  123 

Motor-generator  System 83 

Multiple-voltage  System 

Balancer  Sets  for 82 

Bullock  Electric  Co 78 

Control  of      .    . 83 

Crocker-Wheeler  Co 77 

Efficiency  of 79 

Three-wire 75 

Ward-Leonard,  H 76 

N 
Name  Plate 

Data 22,  281 

Efficiency 22 

Nominal  Rating 282 

Northern  Elec.  Mfg.  Co 55 


Osnos,  M 261 

Osnos  Circle  Diagram 261 


Pacinotti 1,  125 

Page  Motor 1 

Performance 

Specifications 273 

Tests      273 

Pfatischer,  M 49,  60 

Phase 

Splitting 232 

Swinging 161 


Page 

Potter,  W.  B 107 

Power  Factor 

of  Induction  Motors    .    .   179,  194,  203,  231 

of  Repulsion  Motors 262 

of  Series  Motors       242,  247,  254 

of  Synchronous  Motors 140,  160 

Press  Control 87,  270 

Preventive  Leads     .  .  246 


Quarter  Phase 165,  168 


Rated 

Current 11,  12 

Voltage 11 

Rating 

Motors 273 

Series  Motors 107,  282 

Repulsion  Motor 

Anthony -Jackson-Ryan      130 

Circle  Diagram 261 

Compensated 259,  262 

History      130 

Speed  Regulation 262 

Thomson,  Prof.  E 130,257 

Winter-Eichberg 263 

Resistance  Control 

Induction  Motors 199,  207,  212 

Series  Motors 109 

Shunt  Motors 33,  38 

Rheostats 

Continuous  Service 281 

Heating  of 179 

Intermittent  Service 281 

j-      Starting 33,109,199,281 

Ridgway  Motor 48,  55 

Rotary  Field 126,  165,  216,  219 

Rotor  Windings 165,  173,  197 

Ryan,  Prof.  H.  J 48,  130 


Saturation  Factor 29 

Series  Motor,  A.  C 123,  129,  241 

Characteristic  Curves 252 

Circle  Diagrams 250 

Compensated 254 

Construction 242 

Control      255 

Losses  in 242,254 

Power  Factor 242,  247 

Preventive  Leads 246 

Siemens,  Alex 129 

Sparking 245,  254 

Transformer  Action 242,  244 

Vector  Diagram 248 

Voltages  occurring  in 242 

Series  Motor,  D.  C 4,  94 

Constant  Current 94,  114 

Constant  Potential      94 

Characteristic  Curves 94  et  seq. 

Connections      94,  100,  105 

Control  of     .  .109 


290 


INDEX 


Page 
Series  Motor,  D.  C.—  Continued. 

Drum  and  Master  Control 113 

Field  Control Ill 

Gears  for 105 

G.  E.  Type  6oC 99 

Racing  of 100 

Rating  of 107,  282 

Resistance  Control 109 

Series-parallel  Control 109 

Speed-current       94 

Speed-tractive  Effort  .    .    .    : 104 

Torque-current 99 

Torque  per  ampere      100 

Westinghouse  Motor 107 

Shunt  Motor 10,  239 

Adjustable-speed  Types 46,  67 

Armature  Resistance-Speed 25,  38 

Auxiliary  Pole      49,  60 

Brush  Shifting  and  Speed 45 

Bullock  Co 49 

Commutation  Pole 49,  60 

Compensated 48,  55 

Data,  Constant-speed  Types 20 

Double  Armature 90 

Efficiency 22,  52,  56,  59,  65,  71 

Electro-Dynamic  Co '.  49,  60 

Field  Control 46,  68 

Flux  Distribution    .    .    .    .  52,  54,  58,  65,  71 

Heating 26 

Interpolar 61,  63 

Lincoln  Mfg.  Co 72 

Magneto  Electric  Co 52 

Methods  of  Speed  Control.    .     38,46,68,74 

Multiple-voltage  Control 74 

Resistance  Control 38 

Speed-load  Curves  .    .  50,  55,  59,  63,  70,  72 

Starting  Box 33 

Storey  Motor 52 

Stow  Mfg.  Co 68 

Voltage  Variation-Speed 28 

Ward-Leonard,  H.  Controls 74 

Service,  Intermittent 280 

Siemens,  Alex 129 

Single-phase  •  Induction     Motor     (see     Induction 
Motor). 

Slip 127,  169,  194,  232 

Slip-ring  Control      199 

Slip-ring  Rotors       174 

Slip-ring  Resistance 182,  201,  227 

Speed  Classifications r.  ........    .  273 

Speed  Control  of 

Induction  Motors 204 

Repulsion  Motors 263 

Series  Motors 109,  255 

Shunt  Motors 38,  46,  68,  74 

Split  Phase 127,  232 

Squirrel-cage  Rotor 173 

Standardization  Rules 272 

Starting 

Box 33,  281 

Compound  Motors 119 

Induction  Motors 194,  235 

Repulsion  Motors 263 


Page 
Starting  —  Con'inued. 

Series  Motors 109,  255 

Shunt  Motors 33 

Synchronous  Motors 136 

Stator  Windings 169 

Steinmetz,  Dr.  C.  P 129,  232,  234 

Storey  Motor 52 

Stow  Motor 68 

Synchronous  Motor 131 

Action 132,  148 

Balancing  Action 161 

Circle  Diagram 153 

Current-phase  Angle  Curves 153 

Current-power  Curves 157 

Damping  Coils 164 

Driving  Power 151 

History 125 

Hunting 162,  164 

Limit  of  Stability 148,  150 

Maximum  Output 147 

Phase  Relations 141 

Power-factor,  Improvement 142 

Speed  of 132 

Starting  of 136 

Starting  Torque 136 

Super-Excitation      143 

Synchronism 132 

Synchronizing 136 

Torque 143 

V  Curves 159 

Variation  of  Field        142 

of  Power  Factor 143 

Synchroscope 139 


Tandem  Control 207 

Teaser  Control 85 

Temperature  Rise 11,  277 

Limit  of 279 

Tesla,  N 128,  165,  232 

Test  Voltage 11,275 

Thompson,  M.  E 49 

Thompson-Ryan  Motor 49,  55 

Thomson,  Prof.  E 130,  257 

Torque 15 

Tractive  Effort 104 

Train  Lines 113 

Types  of  Motors      4,  123 

Types  of  Service 267 


V  Curves 158 

Vector  Diagrams 

Induction  Motor      184 

Repulsion  Motor 260 

Series  Motor 248 

Shunt  Motor 240 

Synchronous  Motor 142,  150 

Voltage-speed 

Induction  Motor 182,  211 

Repulsion  Motor 263 

Series  Motor 99,  253 

Shunt  Motor    .  .  28,  74 


INDEX 


291 


Voltage  Test 

Page 
.     11,  275 

Voltage-Torque 

.   182,  266 

W 
Wagner  Motor             .       .    .    .    . 

235 

Ward-Leonard  H 

....  76,  83 

Westinghouse  Series  Motor       .    . 

106 

Wheels  . 

.   105 

Page 

Wilde,  Prof 125 

Windings 

Balancing 50,  58 

Compensation 48,  254,  259,  262 

Double-armature 90 

Rotor 165,  173,  197 

Starting 232 

Stator 169 


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BOTTONE,  S.  R.  Magnetos  For  Automobilists,  How  Made  and  How  Used.  A 
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FOSTER,  H.  A.  With  the  Collaboration  of  Eminent  Specialists.  Electrical  Engi- 
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GERHARDI,  C.  H.  W.  Electricity  Meters;  their  Construction  and  Management. 
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GORE,  GEORGE.  The  Art  of  Electrolytic  Separation  of  Metals  (Theoretical  and 
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GRAY,  J.  Electrical  Influence  Machines:  Their  Historical  Development  and 
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HAMMER,  W.  J.  Radium,  and  Other  Radio-Active  Substances;  Polonium,  Actin- 
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HARRISON,  N.  Electric  Wiring  Diagrams  and  Switchboards.  Illustrated.  12mo., 
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HASKINS,  C.  H.  The  Galvanometer  and  its  Uses.  A  Manual  for  Electricians 
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HAY,  ALFRED.  Principles  of  Alternate-Current  Working.  Second  Edition. 
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HOBART,  H.  M.  Heavy  Electrical  Engineering.  Illustrated.  8vo.,  cloth,  338 
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6  LIST  OF  WORKS  ON  ELECTRICAL  SCIENCE. 

HOUSTON,  EDWIN  J.  A  Dictionary  of  Electrical  Words,  Terms,  and  Phrases. 
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A  Pocket  Dictionary  of  Electrical  Words,  Terms,  and  Phrases.     12mo.,  cloth, 
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HUTCHINSON,  R.  W.,  Jr.  Long-Distance  Electric  Power  Transmission:  Being 
a  Treatise  on  the  Hydro-Electric  Generation  of  Energy;  Its  Transformation, 
Transmission,  and  Distribution.  Second  Edition.  Illustrated.  12mo., 
cloth,  350  pp Net,  $3.00 

— —  and  IHLSENG,  M.  C.  Electricity  in  Mining.  Being  a  theoretical  and  prac- 
tical treatise  on  the  construction,  operation,  and  maintenance  of  electrical 
mining  machinery.  Illustrated.  12mo.,  cloth In  Press 

INCANDESCENT  ELECTRIC  LIGHTING.  A  Practical  Description  of  the  Edison 
System,  by  H.  Latimer.  To  which  is  added:  The  Design  and  Operation  of 
Incandescent  Stations,  by  C.  J.  Field;  A  Description  of  the  Edison  Electro- 
lyte Meter,  by  A.  E.  Kennelly;  and  a  Paper  on  the  Maximum  Efficiency  of 
Incandescent  Lamps,  by  T.  W.  Howell.  Fifth  Edition.  Illustrated. 
16mo.,  cloth,  140  pp.  (No.  57  Van  Nostrand's  Science  Series.).  . .  .50  cents 

INDUCTION  COILS:  How  Made  and  How  Used.  Eleventh  Edition.  Illustrated. 
16mo.,  cloth,  123  pp.  (No.  53  Van  Nostrand's  Science  Series.). .  .50  cents 

JEHL,  FRANCIS.  The  Manufacture  of  Carbons  for  Electric  Lighting  and  Other 
Purposes.  Illustrated.  8vo  ,  cloth,  232  pp Net,  $4 . 00 

JONES,  HARRY  C.     The  Electrical  Nature  of  Matter  and  Radioactivity.     Second 
Edition,  completely  revised.     12mo.,  cloth,  212  pp $2. CO 

KAPP,  GISBERT.  Electric  Transmission  of  Energy  and  its  Transformation, 
Subdivision,  and  Distribution.  A  Practical  Handbook.  Fourth  Edition, 
thoroughly  revised.  Illustrated.  12mo.,  cloth,  445  pp $3.50 

KAPP,  GISBERT.     Alternate-Current  Machinery.      Illustrated.      16mo.,     cloth, 

190  pp.     (No.  96  Van  Nostrand's  Science  Series.) 50  cents 

Dynamos,    Alternators,     and    Transformers.     Illustrated.     8vo.,    cloth,    507 
pp $4.00 

KELSEY,  W.  R.  Continuous-Current  Dynamos  and  Motors,  and  their  Control; 
being  a  series  of  articles  reprinted  from  the  "Practical  Engineer,"  and  com- 
pleted by  W  R.  Kelsey,  B.Sc.  With  Tables,  Figures,  and  Diagrams.  8vo., 
cloth,  439  pp , $2.50 

KEMPE,  H.  R.  A  Handbook  of  Electrical  Testing.  Seventh  Edition, 
revised  and  enlarged.  285  Illustrations.  8vo.,  cloth,  706  pp. . .  .Net,  $6.00 

KENNEDY,    R.     Modern    Engines    and    Power    Generators.     Illustrated.     4to., 

Electrical   Installations   of   Electric   Light,  Power,  and   Traction    Machinery. 

Illustrated.     8vo.,  cloth,  5  vols.     The  Set,  $15.00 Each,  $3 . 50 


LIST  OF  WORKS  ON  ELECTRICAL  SCIENCE.  7 

KENNELLY,  A.  E.  Theoretical  Elements  of  Electro-Dynamic  Machinery.  Vol  I. 
Illustrated.  8vo.,  cloth,  90  pp $1 . 50 

KERSHAW,  J.  B.  C.  The  Electric  Furnace  in  Iron  and  Steel  Production.  Illus- 
trated. 8vo.,  cloth,  74  pp Net,  $1 .50 

Electrometallurgy.     Illustrated.     8vo.,  cloth,  303  pp.     (Van  Nostrand's  West- 
minster Series.) Net,  $2 .00 

KINZBRUNNER,  C.  Continuous-Current  Armatures;  their  Winding  and  Con- 
struction. 79  Illustrations.  8vo.,  cloth,  80  pp Net,  $1 .50 

Alternate-Current  Windings;    their  Theory  and  Construction.     89  Illustrations. 
8vo.,  cloth,  80  pp Net,  $1 .50 

KOE 1TER,  F.  Hydroelectric  Developments  and  Engineering.  A  practical  and 
theoretical  treatise  on  the  development,  design,  construction,  equipment  and 
operation  of  hydroelectric  transmission  plants.  500  illustrations.  4to., 
cloth,  475  pp : , ...Net,  $5.00 

—  Steam-Electric  Power  Plants.  A  practical  treatise  011  the  design  of  central 
light  and  power  stations  and  their  economical  construction  and  operation. 
Fully  Illustrated.  4to  ,  cloth,  455  pp Net,  $5 .00 

LARNER,  E.  T.  The  Principles  of  Alternating  Currents  for  Students  of  Electrical 
Engineering.  Illustrated  with  Diagrams.  12mo.,  cloth,  144  pp. Net,  $1.50 

LEMSTROM,  S.  Electricity  in  Agriculture  and  Horticulture.  Illustrated.  8vo., 
cloth Net,  $1 .50 

LIVERMORE,  V.  P.,  and  WILLIAMS,  J.  How  to  Become  a  Competent  Motorman : 
Being  a  practical  treatise  on  the  proper  method  of  operating  a  street-railway 
motor-car;  also  giving  details  how  to  overcome  certain  defects.  Second 
Edition.  Illustrated.  16mo.,  cloth,  247  pp. Net,  $1 .00 

LOCKWOOD,  T.  D.  Electricity,  Magnetism,  and  Electro-Telegraphy.  A  Prac- 
tical Guide  and  Handbook  of  General  Information  for  Electrical  Students, 
Operators,  and  Inspectors.  Fourth  Edition.  Illustrated.  8vo.,  cloth, 
374  pp $2.50 

LODGE,  OLIVER  J.  Signalling  Across  Space  Without  Wires:  Being  a  description 
of  the  work  of  Hertz  and  his  successors.  Fourth  Edition.  Illustrated.  8vo., 
cloth,  156  pp Net,  $2.00 

LORING,  A.  E.  A  Handbook  of  the  Electro-Magnetic  Telegraph.  Fourth  Edition, 
revised.  Illustrated.  16mo.,  cloth,  116  pp.  (No.  39  Van  Nostrand's 
Science  Series.) 50  cents 

LUPTON,  A.  PARR,  G.  D.  A.,  and  PERKIN,  H.  Electricity  Applied  to  Mining. 
Second  Edition.  With  Tables,  Diagrams,  and  Folding  Plates.  8vo..  cloth, 
320  pp Net,  $4.50 

MAILLOUX,     C.     0.     Electric     Traction    Machinery.     Illustrated.     8vo.,     cloth. 

In  Press 


8  LIST  OF  WORKS  ON  ELECTRICAL  SCIENCE. 

MANSFIELD,  A.  K.  Electromagnets:  Their  Design  and  Construction.  Second 
Edition.  Illustrated.  IGmo.,  cloth,  155  pp.  (Van  Nostrand's  Science 
Series  No.  64.) • 50  cents 

MASSIE,  W.  W.,  and  UNDERHILL,  C.  R.  Wireless  Telegraphy  and  Telephony 
Popularly  Explained.  Illustrated.  12mo.,  cloth,  82  pp Net,  $1 .00 

MAURICE,  W.  Electrical  Blasting  Apparatus  and  Explosives,  with  special  refer- 
ence to  colliery  practice.  Illustrated.  8vo.,  cloth,  167  pp Net,  $3.50 

MONCKTON,  C.  C.  F.  Radio  Telegraphy.  173  Illustrations.  Svo.,  cloth,  272  pp. 
(Van  Nostrand's  Westminster  Series.) Net,  $2 .00 

MORGAN,  ALFRED  P.  Wireless  Telegraph  Construction  for  Amateurs.  153  illus- 
trations. 12mo.,  cloth,  220  pp .' Net,  $1.50 

NIPHER,  FRANCIS  E.  Theory  of  Magnetic  Measurements.  With  an  Appendix 
on  the  Method  of  Least  Squares.  Illustrated.  12mo.,  cloth,  94  pp.  .  .$1 .00 

NOLL,  AUGUSTUS.  How  to  Wire  Buildings.  A  Manual  of  the  Art  of  Interior 
Wiring.  Fourth  Edition.  Illustrated.  12mo.,  cloth,  165  pp $1 .50 

OHM,  G.  S.  The  Galvanic  Circuit  Investigated  Mathematically.  Berlin,  1827. 
Translated  by  William  Francis.  With  Preface  and  Notes  by  the  Editor, 
Thos.  D.  Lockwood.  Second  Edition.  Illustrated.  16mo.,  cloth,  269  pp. 
(No.  102  Van  Nostrand's  Science  Series.) ' 50  cents 

OLSSON,  ANDREW.  Motor  Control  as  Used  in  Connection  with  Turret  Turning  and 
Gun  Elevating  (The  Ward  Leonard  System.)  13  illustrations.  12mo., 
paper,  27  pp.  (U.  S.  Navy  Electrical  Series  No.  1.) Net,  .50 

OUDIN,  MAURICE  A.  Standard  Polyphase  Apparatus  and  Systems.  Illustrated 
with  many  Photo-reproductions,  Diagrams,  and  Tables.  Fifth  Edition,  revised. 
8vo.,  cloth,  369  pp Net,  $3.00 

PALAZ,  A.  Treatise  on  Industrial  Photometry.  Specially  applied  to  Electric 
Lighting.  Translated  from  the  French  by  G.  W.  Patterson,  Jr.,  Assistant 
Professor  of  Physics  in  the  University  of  Michigan,  and  M.  R.  Patterson, 
B.A.  Second  Edition.  Fully  Illustrated.  8vo.,  cloth,  324  pp $4.00 

PARR,  G.  D.  A.  Electrical  Engineering  Measuring  Instruments  for  Commercial 
and  Laboratory  Purposes.  With  370  Diagrams  and  Engravings.  8vo., 
cloth,  328  pp Net,  $3 . 50 

PARSHALL,  H.  F.,  and  HOBART,  H.  M.  Armature  Windings  of  Electric  Machines. 
Third  Edition.  With  140  full-page  Plates,  65  Tables,  and  165  pages  of 

descriptive  letter-press.     4to.,  cloth,  300  pp $7.50 

Electric   Railway  Engineering.     With  437   Figures   and   Diagrams   and  many 
Tables.     4to.,  cloth,  475  pp Net,  $10.00 

Electric  Machine  Design.     Being  a  revised  and  enlarged  edition  of  "Electric 
Generators."     648  Illustrations.     4to,  half  morocco,  601  pp Net,  $12 . 50 


LIST  OF  WORKS  ON  ELECTRICAL  SCIENCE.  9 

PERRINE,  F.  A.  C.  Conductors  for  Electrical  Distribution:  Their  Manufacture 
and  Materials,  the  Calculation  of  Circuits,  Pole-Line  Construction,  Under- 
ground Working,  and  other  Uses.  Second  Edition.  Illustrated.  8vo., 
cloth,  287  pp.  .  T. Net,  $3.50 

POPE,  F.  L.  Modern  Practice  of  the  Electric  Telegraph.  A  Handbook  for  Elec- 
tricians and  Operators.  Seventeenth  Edition.  Illustrated.  8vo.,  cloth, 
234  pp .$1.50 

RAPHAEL,  F.  C.  Localization  of  Faults  in  Electric-Light  Mains.  Second  Edition, 
revised.  Illustrated.  8vo.,  cloth,  205  pp Net,  $3 .00 

RAYMOND,  E.  B.  Alternating-Current  Engineering,  Practically  Treated.  Third 
Edition,  revised.  With  many  Figures  and  Diagrams.  8vo.,  cloth,  244  pp., 

Net,  $2.50 

RICHARDSON,  S.  S.  Magnetism  and  Electricity  and  the  Principles  of  Electrical 
Measurement.  254  illustrations.  12mo. ,  cloth,  604  pp Net,  $2 . 00 

ROBERTS,  J.  Laboratory  Work  in  Electrical  Engineering — Preliminary  Grade. 
A  series  of  laboratory  experiments  for  first-  and  second-year  students  in 
electrical  engineering.  Illustrated  with  many  Diagrams.  8vo.,  cloth, 
218  pp Net,  $2.00 

RUHMER,  ERNST.  Wireless  Telephony  in  Theory  and  Practice.  Translated 
from  the  German  by  James  Erskine-Murray.  Illustrated.  8vo.,  cloth, 
224  pp Net,  $3.50 

RUSSELL,  A.  The  Theory  of  Electric  Cables  and  Networks.  71  Illustrations. 
8vo.,  cloth,  275  pp Net,  $3 .00 

SALOMONS,  DAVID.  Electric-Light  Installations.  A  Practical  Handbook.  Illus- 
trated. 12mo.,  cloth. 

Vol  I.:    Management  of  Accumulators.     Ninth  Edition.     178  pp $2.50 

Vol.  II.:    Apparatus.     Seventh  Edition.     318  pp $2 .25 

Vol.  III.:    Application.     Seventh  Editwn.     234  pp $1 .50 

SEVER,  G.  F.  Electrical  Engineering  Experiments  and  Tests  on  Direct-Current 
Machinery.  Second  Edition,  enlarged.  WTith  Diagrams  and  Figures.  8vo., 
pamphlet,  75  pp Net,  $1 .00 

-  and  TOWNSEND,  F.     Laboratory  and  Factory  Tests  in  Electrical  Engineering. 

Second  Edition.     Illustrated.     8vo.,  cloth,  269  pp Net,  $2 . 50 

SEW  ALL,   C.   H.     Wireless  Telegraphy.     With   Diagrams   and   Figures.     Second 

Edition,  corrected.     Illustrated.     8vo.,  cloth,  229  pp Net,  $2.00 

Lessons  in  Telegraphy.     Illustrated.     12mo.,  cloth,  104  pp Net,  $1 .00 

T.     Elements  of  Electrical  Engineering.     Third  Edition,  revised.    Illustrated. 

8vo.,  cloth,  444  pp $3.00 

The  Construction  of  Dynamos  (Alternating  and  Direct  Current).  A  Text- 
book for  students,  engineering  contractors,  and  electricians-in-charge. 
Illustrated.  8vo.,  cloth,  316  pp $3.00 


10  LIST  OF  WORKS  ON  ELECTRICAL  SCIENCE. 

SHELDON,  S.,  and  HAUSMANN,  E.  Dynamo-Electric  Machinery :    Its  Construction, 

Design,  and  Operation. 

Vol.  I.:  Direct-Current  Machines.  Eighth  Edition,  completely  rewritten. 
Illustrated.  8vo.,  cloth,  281  pp Net,  $2 .50 

and  MASON,  H.     Alternating-Current  Machines :  Being  the  second  volume 

of  "  Dynamo  -Electric  Machinery;  its  Construction,  Design,  and  Opera- 
tion." With  many  Diagrams  and  Figures.  (Binding  uniform  with  Vol- 
ume I.)  Seventh  Edition,  rewritten.  8vo.,  cloth;  353  pp Net,  $2 .50 

SLOANE,  T.  O'CONOR.  Standard  Electrical  Dictionary.  300  Illustrations.  12mo., 
cloth,  682  pp $3 .00 

Elementary  Electrical  Calculations.  A  Manual  of  Simple  Engineering 
Mathematics,  covering  the  whole  field  of  Direct  Current  Calculations,  the 
basis  of  Alternating  Current  Mathematics,  Networks,  and  typical  cases  of 
Circuits,  with  Appendices  on  special  subjects.  8vo.,  cloth.  Illustrated. 
304  pp '. Net,  $2.00 

SNELL,  ALBION  T.  Electric  Motive  Power.  The  Transmission  and  Distribution 
of  Electric  Power  by  Continuous  and  Alternating  Currents.  With  a  Section 
on  the  Applications  of  Electricity  to  Mining  Work.  Second  Edition. 
Illustrated.  8vo.,  cloth,  411  pp Net,  $4.00 

SODDY,  F.  Radio-Activity ;  an  Elementary  Treatise  from  the  Standpoint  of  the 
Disintegration  Theory.  Fully  Illustrated.  8vo.,  cloth,  214  pp.  .Net,  $3.00 

SOLOMON,  MAURICE.  Electric  Lamps.  Illustrated.  8vo.,  cloth.  (Van  Nos- 
trand's Westminster  Series.) Net,  $2 . 00 

STEWART,  A.  Modern  Polyphase  Machinery.  Illustrated.  12mo.,  cloth,  290 
pp Net,  $2.00 

SWINBURNE,  JAS.,  and  WORDINGHAM,  C.  H.  The  Measurement  of  Electric 
Currents.  Electrical  Measuring  Instruments.  Meters  for  Electrical  Energy. 
Edited,  with  Preface,  by  T.  Commerford  Martin.  Folding  Plate  and  Numer- 
ous Illustrations.  16mo.,  cloth,  241  pp.  (No.  109  Van  Nostrand's  Science 
Series.) 50  cents 

SWOOPE,  C.  WALTON.  Lessons  in  Practical  Electricity:  Principles,  Experi- 
ments, and  Arithmetical  Problems.  An  Elementary  Text-book.  Eleventh 
Edition,  enlarged  with  a  chapter  on  alternating  currents.  404  illustrations. 
12mo.,  cloth,  507  pp Net,  $2.00 

THOM,  C.,  and  JONES,  W.  H.  Telegraphic  Connections,  embracing  recent  methods 
in  Quadruplex  Telegraphy.  20  Colored  Plates.  8vo.,  cloth,  59  pp.  .$1-50 

THOMPSON,  S.  P.,  Prof.  Dynamo-Electric  Machinery.  With  an  Introduction 
and  Notes  by  Frank  L.  Pope  and  H.  R.  Butler.  Fully  Illustrated.  16mo., 
cloth,  214  pp.  (No.  66  Van  Nostrand's  Science  Series.) 50  cents 

Recent  Progress  in  Dynamo-Electric  Machines.  Being  a  Supplement  to 
"Dynamo-Electric  Machinery."  Illustrated.  16mo.,  cloth,  113  pp.  (No. 
75  Van  Nostrand's  Science  Series.) 50  cents 


LIST  OF   WORKS  ON  ELECTRICAL  SCIENCE.  11 

TOWNSEND,  FITZHUGH.  Alternating  Current  Engineering.  Illustrated.  8vo.; 
paper,  32  pp Net,  75  cents 

UNDERHILL,  C.  R.  Solenoids,  Electromagnets  and  Electromagnetic  Windings. 
218  Illustrations.  12mo.,  cloth,  345  pp Net,  $2.00 

URQUHART,  J.  W.  Dynamo  Construction.  A  Practical  Handbook  for  the  use 
of  Engineer  Constructors  and  Electricians  in  Charge.  Illustrated.  12mo.. 
cloth $3.00 

Electric  Ship-Lighting.  A  Handbook  on  the  Practical  Fitting  and  Running  of 
Ship's  Electrical  Plant,  for  the  use  of  Ship  Owners  and  Builders,  Marine 
Electricians,  and  Sea-going  Engineers  in  Charge.  88  Illustrations.  12mo., 
cloth,  308  pp $3.00 

Electric-Light  Fitting.  A  Handbook  for  Working  Electrical  Engineers,  em- 
bodying Practical  Notes  on  Installation  Management.  Second  Edition. 
With  numerous  Illustrations.  12mo.,  cloth $2 .00 

Electroplating.    Fifth  Edition.     Illustrated.     12mo.,  cloth,  230  pp $2.00 

Electrotyping.     Illustrated.     12mo.,  cloth,  228  pp $2 .00 

WADE,  E.  J.  Secondary  Batteries:  Their  Theory,  Construction,  and  Use.  Second 
Edition,  corrected.  265  illustrations.  8vo,  cloth,  501  pp Net,  $4.00 

WADSWORTH,  C.     Electric  Ignition.     15  Illustrations.     16mo.     paper.     In  Press 

WALKER,  FREDERICK.  Practical  Dynamo-Building  for  Amateurs.  How  to 
Wind  for  any  Output.  Third  Edition.  Illustrated.  16mo.,  cloth,  104  pp. 
(No.  98  Van  Nostrand's  Science  Series.) 50  cents 

—  SYDNEY  F.  Electricity  in  Homes  and  Workshops.  A  Practical  Treatise  on 
Auxiliary  Electrical  Apparatus.  Fourth  Edition.  Illustrated.  12mo., 
cloth,  358  pp $2 .00 

Electricity  in  Mining.     Illustrated.     8vo.,  cloth,  385  pp $3 .50 

WALLING,  B.  T.,  Lieut.-Com.  U.S.N.,  and  MARTIN,  JULIUS.  Electrical  Installa- 
tions of  the  United  States  Navy.  With  many  Diagrams  and  Engravings. 
8vo.,  cloth,  648  pp $6.00 

WATT,    ALEXANDER.     Electroplating    and    Refining    of    Metals.     New    Edition, 

rewritten  by  Arnold  Philip.     Illustrated.     8vo.,  cloth,  677  pp.  .Net,  $4.50 

Electro-Metallurgy.     Fifteenth   Edition.     Illustrated.     12mo.,    cloth,    225    pp., 

$1.00 

WEBB,  H.  L.  A  Practical  Guide  to  the  Testing  of  Insulated  Wires  and  Cables. 
Fifth  Edition.  Illustrated.  12mo.,  cloth,  118  pp $1 .00 

WEEKS,  R.  W.    The  Design  of  Alternate-Current  Transformer.     New  Edition 

in  Press 


12  ,  LIST  OF  WORKS  ON  ELECTRICAL  SCIENCE. 

WEYMOUTH,  F.  MARTEN.  Drum  Armatures  and  Commutators.  (Theory  and 
Practice.)  A  complete  treatise  on  the  theory  and  construction  of  drum- 
winding,  and  of  commutators  for  closed-coil  armatures,  together  with  a  full 
resume"  of  some  of  the  principal  points  involved  in  their  design,  and  an 
exposition  of  armature  reactions  and  sparking.  Illustrated.  8vo.,  cloth, 
295 pp Net,  $3.00 

WILKINSON,  H.  D.  Submarine  Cable-Laying,  Repairing,  and  Testing.  New  Edition . 
Illustrated.  8vo.,  cloth In  Frets 

YOUNG,  J.  ELTON.  Electrical  Testing  for  Telegraph  Engineers.  Illustrated. 
8vo.,  cloth,  264  pp Net,  $4.00 

ZEIDLER,  J.  and  LUSTGARTEN,  J.  Electric  Arc  Lamps.  Their  principles,  con- 
struction and  working.  160  illustrations.  8vo.,  cloth,  200  pp Net,  $2.00 


A  96=page  Catalog  of  Books  on  Electricity,  classified  by 
subjects,  will  be  furnished  gratis,  postage  prepaid,  on 
application*  *k 


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